diff --git a/.gitignore b/.gitignore
index c00e966..32c2fec 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,16 +1,11 @@
 *.egg-info
-*.iml
 *.py[co]
 .*.sw[a-z]
-.pytest_cache
 .coverage
-.idea
-.project
-.pydevproject
 .tox
 .venv.touch
+/.mypy_cache
+/.pytest_cache
 /venv*
 coverage-html
 dist
-# SublimeText project/workspace files
-*.sublime-*
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 8bd0fdc..4990537 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -27,7 +27,7 @@
     rev: v1.3.5
     hooks:
     -   id: reorder-python-imports
-        language_version: python2.7
+        language_version: python3
 -   repo: https://github.com/asottile/pyupgrade
     rev: v1.11.1
     hooks:
@@ -36,3 +36,8 @@
     rev: v0.7.1
     hooks:
     -   id: add-trailing-comma
+-   repo: https://github.com/pre-commit/mirrors-mypy
+    rev: v0.660
+    hooks:
+    -   id: mypy
+        language_version: python3
diff --git a/.travis.yml b/.travis.yml
index 477b5c4..fa16cce 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,4 @@
+dist: xenial
 language: python
 matrix:
     include:  # These should match the tox env list
@@ -6,9 +7,8 @@
         python: 3.6
     -   env: TOXENV=py37
         python: 3.7
-        dist: xenial
     -   env: TOXENV=pypy
-        python: pypy-5.7.1
+        python: pypy2.7-5.10.0
 install: pip install coveralls tox
 script: tox
 before_install:
diff --git a/get-git-lfs.py b/get-git-lfs.py
index 48dd31e..4b09cac 100755
--- a/get-git-lfs.py
+++ b/get-git-lfs.py
@@ -4,7 +4,9 @@
 import os.path
 import shutil
 import tarfile
-from urllib.request import urlopen
+import urllib.request
+from typing import cast
+from typing import IO
 
 DOWNLOAD_PATH = (
     'https://github.com/github/git-lfs/releases/download/'
@@ -15,7 +17,7 @@
 DEST_DIR = os.path.dirname(DEST_PATH)
 
 
-def main():
+def main():  # type: () -> int
     if (
             os.path.exists(DEST_PATH) and
             os.path.isfile(DEST_PATH) and
@@ -27,12 +29,13 @@
     shutil.rmtree(DEST_DIR, ignore_errors=True)
     os.makedirs(DEST_DIR, exist_ok=True)
 
-    contents = io.BytesIO(urlopen(DOWNLOAD_PATH).read())
+    contents = io.BytesIO(urllib.request.urlopen(DOWNLOAD_PATH).read())
     with tarfile.open(fileobj=contents) as tar:
-        with tar.extractfile(PATH_IN_TAR) as src_file:
+        with cast(IO[bytes], tar.extractfile(PATH_IN_TAR)) as src_file:
             with open(DEST_PATH, 'wb') as dest_file:
                 shutil.copyfileobj(src_file, dest_file)
     os.chmod(DEST_PATH, 0o755)
+    return 0
 
 
 if __name__ == '__main__':
diff --git a/mypy.ini b/mypy.ini
new file mode 100644
index 0000000..ee62c89
--- /dev/null
+++ b/mypy.ini
@@ -0,0 +1,12 @@
+[mypy]
+check_untyped_defs = true
+disallow_any_generics = true
+disallow_incomplete_defs = true
+disallow_untyped_defs = true
+no_implicit_optional = true
+
+[mypy-testing.*]
+disallow_untyped_defs = false
+
+[mypy-tests.*]
+disallow_untyped_defs = false
diff --git a/pre_commit_hooks/autopep8_wrapper.py b/pre_commit_hooks/autopep8_wrapper.py
index 9951924..8b69a04 100644
--- a/pre_commit_hooks/autopep8_wrapper.py
+++ b/pre_commit_hooks/autopep8_wrapper.py
@@ -3,7 +3,7 @@
 from __future__ import unicode_literals
 
 
-def main(argv=None):
+def main():  # type: () -> int
     raise SystemExit(
         'autopep8-wrapper is deprecated.  Instead use autopep8 directly via '
         'https://github.com/pre-commit/mirrors-autopep8',
diff --git a/pre_commit_hooks/check_added_large_files.py b/pre_commit_hooks/check_added_large_files.py
index 2d06706..be39498 100644
--- a/pre_commit_hooks/check_added_large_files.py
+++ b/pre_commit_hooks/check_added_large_files.py
@@ -7,13 +7,17 @@
 import json
 import math
 import os
+from typing import Iterable
+from typing import Optional
+from typing import Sequence
+from typing import Set
 
 from pre_commit_hooks.util import added_files
 from pre_commit_hooks.util import CalledProcessError
 from pre_commit_hooks.util import cmd_output
 
 
-def lfs_files():
+def lfs_files():  # type: () -> Set[str]
     try:
         # Introduced in git-lfs 2.2.0, first working in 2.2.1
         lfs_ret = cmd_output('git', 'lfs', 'status', '--json')
@@ -24,6 +28,7 @@
 
 
 def find_large_added_files(filenames, maxkb):
+    # type: (Iterable[str], int) -> int
     # Find all added files that are also in the list of files pre-commit tells
     # us about
     filenames = (added_files() & set(filenames)) - lfs_files()
@@ -38,7 +43,7 @@
     return retv
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument(
         'filenames', nargs='*',
diff --git a/pre_commit_hooks/check_ast.py b/pre_commit_hooks/check_ast.py
index ded65e4..0df3540 100644
--- a/pre_commit_hooks/check_ast.py
+++ b/pre_commit_hooks/check_ast.py
@@ -7,9 +7,11 @@
 import platform
 import sys
 import traceback
+from typing import Optional
+from typing import Sequence
 
 
-def check_ast(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*')
     args = parser.parse_args(argv)
@@ -34,4 +36,4 @@
 
 
 if __name__ == '__main__':
-    exit(check_ast())
+    exit(main())
diff --git a/pre_commit_hooks/check_builtin_literals.py b/pre_commit_hooks/check_builtin_literals.py
index 4a4b9ce..874c68c 100644
--- a/pre_commit_hooks/check_builtin_literals.py
+++ b/pre_commit_hooks/check_builtin_literals.py
@@ -4,6 +4,10 @@
 import ast
 import collections
 import sys
+from typing import List
+from typing import Optional
+from typing import Sequence
+from typing import Set
 
 
 BUILTIN_TYPES = {
@@ -22,14 +26,17 @@
 
 class BuiltinTypeVisitor(ast.NodeVisitor):
     def __init__(self, ignore=None, allow_dict_kwargs=True):
-        self.builtin_type_calls = []
+        # type: (Optional[Sequence[str]], bool) -> None
+        self.builtin_type_calls = []  # type: List[BuiltinTypeCall]
         self.ignore = set(ignore) if ignore else set()
         self.allow_dict_kwargs = allow_dict_kwargs
 
-    def _check_dict_call(self, node):
+    def _check_dict_call(self, node):  # type: (ast.Call) -> bool
+
         return self.allow_dict_kwargs and (getattr(node, 'kwargs', None) or getattr(node, 'keywords', None))
 
-    def visit_Call(self, node):
+    def visit_Call(self, node):  # type: (ast.Call) -> None
+
         if not isinstance(node.func, ast.Name):
             # Ignore functions that are object attributes (`foo.bar()`).
             # Assume that if the user calls `builtins.list()`, they know what
@@ -47,6 +54,7 @@
 
 
 def check_file_for_builtin_type_constructors(filename, ignore=None, allow_dict_kwargs=True):
+    # type: (str, Optional[Sequence[str]], bool) -> List[BuiltinTypeCall]
     with open(filename, 'rb') as f:
         tree = ast.parse(f.read(), filename=filename)
     visitor = BuiltinTypeVisitor(ignore=ignore, allow_dict_kwargs=allow_dict_kwargs)
@@ -54,24 +62,22 @@
     return visitor.builtin_type_calls
 
 
-def parse_args(argv):
-    def parse_ignore(value):
-        return set(value.split(','))
+def parse_ignore(value):  # type: (str) -> Set[str]
+    return set(value.split(','))
 
+
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*')
     parser.add_argument('--ignore', type=parse_ignore, default=set())
 
-    allow_dict_kwargs = parser.add_mutually_exclusive_group(required=False)
-    allow_dict_kwargs.add_argument('--allow-dict-kwargs', action='store_true')
-    allow_dict_kwargs.add_argument('--no-allow-dict-kwargs', dest='allow_dict_kwargs', action='store_false')
-    allow_dict_kwargs.set_defaults(allow_dict_kwargs=True)
+    mutex = parser.add_mutually_exclusive_group(required=False)
+    mutex.add_argument('--allow-dict-kwargs', action='store_true')
+    mutex.add_argument('--no-allow-dict-kwargs', dest='allow_dict_kwargs', action='store_false')
+    mutex.set_defaults(allow_dict_kwargs=True)
 
-    return parser.parse_args(argv)
+    args = parser.parse_args(argv)
 
-
-def main(argv=None):
-    args = parse_args(argv)
     rc = 0
     for filename in args.filenames:
         calls = check_file_for_builtin_type_constructors(
diff --git a/pre_commit_hooks/check_byte_order_marker.py b/pre_commit_hooks/check_byte_order_marker.py
index 1541b30..10667c3 100644
--- a/pre_commit_hooks/check_byte_order_marker.py
+++ b/pre_commit_hooks/check_byte_order_marker.py
@@ -3,9 +3,11 @@
 from __future__ import unicode_literals
 
 import argparse
+from typing import Optional
+from typing import Sequence
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*', help='Filenames to check')
     args = parser.parse_args(argv)
diff --git a/pre_commit_hooks/check_case_conflict.py b/pre_commit_hooks/check_case_conflict.py
index 0f78296..e343d61 100644
--- a/pre_commit_hooks/check_case_conflict.py
+++ b/pre_commit_hooks/check_case_conflict.py
@@ -3,16 +3,20 @@
 from __future__ import unicode_literals
 
 import argparse
+from typing import Iterable
+from typing import Optional
+from typing import Sequence
+from typing import Set
 
 from pre_commit_hooks.util import added_files
 from pre_commit_hooks.util import cmd_output
 
 
-def lower_set(iterable):
+def lower_set(iterable):  # type: (Iterable[str]) -> Set[str]
     return {x.lower() for x in iterable}
 
 
-def find_conflicting_filenames(filenames):
+def find_conflicting_filenames(filenames):  # type: (Sequence[str]) -> int
     repo_files = set(cmd_output('git', 'ls-files').splitlines())
     relevant_files = set(filenames) | added_files()
     repo_files -= relevant_files
@@ -41,7 +45,7 @@
     return retv
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument(
         'filenames', nargs='*',
diff --git a/pre_commit_hooks/check_docstring_first.py b/pre_commit_hooks/check_docstring_first.py
index 9988378..f4639f1 100644
--- a/pre_commit_hooks/check_docstring_first.py
+++ b/pre_commit_hooks/check_docstring_first.py
@@ -5,6 +5,8 @@
 import argparse
 import io
 import tokenize
+from typing import Optional
+from typing import Sequence
 
 
 NON_CODE_TOKENS = frozenset((
@@ -13,6 +15,7 @@
 
 
 def check_docstring_first(src, filename='<unknown>'):
+    # type: (str, str) -> int
     """Returns nonzero if the source has what looks like a docstring that is
     not at the beginning of the source.
 
@@ -50,7 +53,7 @@
     return 0
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*')
     args = parser.parse_args(argv)
diff --git a/pre_commit_hooks/check_executables_have_shebangs.py b/pre_commit_hooks/check_executables_have_shebangs.py
index 89ac6e5..c936a5d 100644
--- a/pre_commit_hooks/check_executables_have_shebangs.py
+++ b/pre_commit_hooks/check_executables_have_shebangs.py
@@ -6,9 +6,11 @@
 import argparse
 import pipes
 import sys
+from typing import Optional
+from typing import Sequence
 
 
-def check_has_shebang(path):
+def check_has_shebang(path):  # type: (str) -> int
     with open(path, 'rb') as f:
         first_bytes = f.read(2)
 
@@ -27,7 +29,7 @@
         return 0
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser(description=__doc__)
     parser.add_argument('filenames', nargs='*')
     args = parser.parse_args(argv)
@@ -38,3 +40,7 @@
         retv |= check_has_shebang(filename)
 
     return retv
+
+
+if __name__ == '__main__':
+    exit(main())
diff --git a/pre_commit_hooks/check_json.py b/pre_commit_hooks/check_json.py
index b403f4b..b939350 100644
--- a/pre_commit_hooks/check_json.py
+++ b/pre_commit_hooks/check_json.py
@@ -4,9 +4,11 @@
 import io
 import json
 import sys
+from typing import Optional
+from typing import Sequence
 
 
-def check_json(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*', help='JSON filenames to check.')
     args = parser.parse_args(argv)
@@ -22,4 +24,4 @@
 
 
 if __name__ == '__main__':
-    sys.exit(check_json())
+    sys.exit(main())
diff --git a/pre_commit_hooks/check_merge_conflict.py b/pre_commit_hooks/check_merge_conflict.py
index 6db5efe..74e4ae1 100644
--- a/pre_commit_hooks/check_merge_conflict.py
+++ b/pre_commit_hooks/check_merge_conflict.py
@@ -2,6 +2,9 @@
 
 import argparse
 import os.path
+from typing import Optional
+from typing import Sequence
+
 
 CONFLICT_PATTERNS = [
     b'<<<<<<< ',
@@ -12,7 +15,7 @@
 WARNING_MSG = 'Merge conflict string "{0}" found in {1}:{2}'
 
 
-def is_in_merge():
+def is_in_merge():  # type: () -> int
     return (
         os.path.exists(os.path.join('.git', 'MERGE_MSG')) and
         (
@@ -23,7 +26,7 @@
     )
 
 
-def detect_merge_conflict(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*')
     parser.add_argument('--assume-in-merge', action='store_true')
@@ -47,4 +50,4 @@
 
 
 if __name__ == '__main__':
-    exit(detect_merge_conflict())
+    exit(main())
diff --git a/pre_commit_hooks/check_symlinks.py b/pre_commit_hooks/check_symlinks.py
index 010c871..736bf99 100644
--- a/pre_commit_hooks/check_symlinks.py
+++ b/pre_commit_hooks/check_symlinks.py
@@ -4,9 +4,11 @@
 
 import argparse
 import os.path
+from typing import Optional
+from typing import Sequence
 
 
-def check_symlinks(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser(description='Checks for broken symlinks.')
     parser.add_argument('filenames', nargs='*', help='Filenames to check')
     args = parser.parse_args(argv)
@@ -25,4 +27,4 @@
 
 
 if __name__ == '__main__':
-    exit(check_symlinks())
+    exit(main())
diff --git a/pre_commit_hooks/check_vcs_permalinks.py b/pre_commit_hooks/check_vcs_permalinks.py
index f0dcf5b..f6e2a7d 100644
--- a/pre_commit_hooks/check_vcs_permalinks.py
+++ b/pre_commit_hooks/check_vcs_permalinks.py
@@ -5,6 +5,8 @@
 import argparse
 import re
 import sys
+from typing import Optional
+from typing import Sequence
 
 
 GITHUB_NON_PERMALINK = re.compile(
@@ -12,7 +14,7 @@
 )
 
 
-def _check_filename(filename):
+def _check_filename(filename):  # type: (str) -> int
     retv = 0
     with open(filename, 'rb') as f:
         for i, line in enumerate(f, 1):
@@ -24,7 +26,7 @@
     return retv
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*')
     args = parser.parse_args(argv)
diff --git a/pre_commit_hooks/check_xml.py b/pre_commit_hooks/check_xml.py
index a4c11a5..66e10ba 100644
--- a/pre_commit_hooks/check_xml.py
+++ b/pre_commit_hooks/check_xml.py
@@ -5,10 +5,12 @@
 import argparse
 import io
 import sys
-import xml.sax
+import xml.sax.handler
+from typing import Optional
+from typing import Sequence
 
 
-def check_xml(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*', help='XML filenames to check.')
     args = parser.parse_args(argv)
@@ -17,7 +19,7 @@
     for filename in args.filenames:
         try:
             with io.open(filename, 'rb') as xml_file:
-                xml.sax.parse(xml_file, xml.sax.ContentHandler())
+                xml.sax.parse(xml_file, xml.sax.handler.ContentHandler())
         except xml.sax.SAXException as exc:
             print('{}: Failed to xml parse ({})'.format(filename, exc))
             retval = 1
@@ -25,4 +27,4 @@
 
 
 if __name__ == '__main__':
-    sys.exit(check_xml())
+    sys.exit(main())
diff --git a/pre_commit_hooks/check_yaml.py b/pre_commit_hooks/check_yaml.py
index 208737f..b638684 100644
--- a/pre_commit_hooks/check_yaml.py
+++ b/pre_commit_hooks/check_yaml.py
@@ -3,22 +3,26 @@
 import argparse
 import collections
 import sys
+from typing import Any
+from typing import Generator
+from typing import Optional
+from typing import Sequence
 
 import ruamel.yaml
 
 yaml = ruamel.yaml.YAML(typ='safe')
 
 
-def _exhaust(gen):
+def _exhaust(gen):  # type: (Generator[str, None, None]) -> None
     for _ in gen:
         pass
 
 
-def _parse_unsafe(*args, **kwargs):
+def _parse_unsafe(*args, **kwargs):  # type: (*Any, **Any) -> None
     _exhaust(yaml.parse(*args, **kwargs))
 
 
-def _load_all(*args, **kwargs):
+def _load_all(*args, **kwargs):  # type: (*Any, **Any) -> None
     _exhaust(yaml.load_all(*args, **kwargs))
 
 
@@ -31,7 +35,7 @@
 }
 
 
-def check_yaml(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument(
         '-m', '--multi', '--allow-multiple-documents', action='store_true',
@@ -63,4 +67,4 @@
 
 
 if __name__ == '__main__':
-    sys.exit(check_yaml())
+    sys.exit(main())
diff --git a/pre_commit_hooks/debug_statement_hook.py b/pre_commit_hooks/debug_statement_hook.py
index 5d32277..02dd3b2 100644
--- a/pre_commit_hooks/debug_statement_hook.py
+++ b/pre_commit_hooks/debug_statement_hook.py
@@ -5,6 +5,9 @@
 import ast
 import collections
 import traceback
+from typing import List
+from typing import Optional
+from typing import Sequence
 
 
 DEBUG_STATEMENTS = {'pdb', 'ipdb', 'pudb', 'q', 'rdb'}
@@ -12,21 +15,21 @@
 
 
 class DebugStatementParser(ast.NodeVisitor):
-    def __init__(self):
-        self.breakpoints = []
+    def __init__(self):  # type: () -> None
+        self.breakpoints = []  # type: List[Debug]
 
-    def visit_Import(self, node):
+    def visit_Import(self, node):  # type: (ast.Import) -> None
         for name in node.names:
             if name.name in DEBUG_STATEMENTS:
                 st = Debug(node.lineno, node.col_offset, name.name, 'imported')
                 self.breakpoints.append(st)
 
-    def visit_ImportFrom(self, node):
+    def visit_ImportFrom(self, node):  # type: (ast.ImportFrom) -> None
         if node.module in DEBUG_STATEMENTS:
             st = Debug(node.lineno, node.col_offset, node.module, 'imported')
             self.breakpoints.append(st)
 
-    def visit_Call(self, node):
+    def visit_Call(self, node):  # type: (ast.Call) -> None
         """python3.7+ breakpoint()"""
         if isinstance(node.func, ast.Name) and node.func.id == 'breakpoint':
             st = Debug(node.lineno, node.col_offset, node.func.id, 'called')
@@ -34,7 +37,7 @@
         self.generic_visit(node)
 
 
-def check_file(filename):
+def check_file(filename):  # type: (str) -> int
     try:
         with open(filename, 'rb') as f:
             ast_obj = ast.parse(f.read(), filename=filename)
@@ -58,7 +61,7 @@
     return int(bool(visitor.breakpoints))
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*', help='Filenames to run')
     args = parser.parse_args(argv)
diff --git a/pre_commit_hooks/detect_aws_credentials.py b/pre_commit_hooks/detect_aws_credentials.py
index ecd9d40..3c87d11 100644
--- a/pre_commit_hooks/detect_aws_credentials.py
+++ b/pre_commit_hooks/detect_aws_credentials.py
@@ -3,11 +3,16 @@
 
 import argparse
 import os
+from typing import Dict
+from typing import List
+from typing import Optional
+from typing import Sequence
+from typing import Set
 
 from six.moves import configparser
 
 
-def get_aws_credential_files_from_env():
+def get_aws_credential_files_from_env():  # type: () -> Set[str]
     """Extract credential file paths from environment variables."""
     files = set()
     for env_var in (
@@ -19,7 +24,7 @@
     return files
 
 
-def get_aws_secrets_from_env():
+def get_aws_secrets_from_env():  # type: () -> Set[str]
     """Extract AWS secrets from environment variables."""
     keys = set()
     for env_var in (
@@ -30,7 +35,7 @@
     return keys
 
 
-def get_aws_secrets_from_file(credentials_file):
+def get_aws_secrets_from_file(credentials_file):  # type: (str) -> Set[str]
     """Extract AWS secrets from configuration files.
 
     Read an ini-style configuration file and return a set with all found AWS
@@ -62,6 +67,7 @@
 
 
 def check_file_for_aws_keys(filenames, keys):
+    # type: (Sequence[str], Set[str]) -> List[Dict[str, str]]
     """Check if files contain AWS secrets.
 
     Return a list of all files containing AWS secrets and keys found, with all
@@ -82,7 +88,7 @@
     return bad_files
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='+', help='Filenames to run')
     parser.add_argument(
@@ -111,7 +117,7 @@
     # of files to to gather AWS secrets from.
     credential_files |= get_aws_credential_files_from_env()
 
-    keys = set()
+    keys = set()  # type: Set[str]
     for credential_file in credential_files:
         keys |= get_aws_secrets_from_file(credential_file)
 
diff --git a/pre_commit_hooks/detect_private_key.py b/pre_commit_hooks/detect_private_key.py
index c8ee961..d31957d 100644
--- a/pre_commit_hooks/detect_private_key.py
+++ b/pre_commit_hooks/detect_private_key.py
@@ -2,6 +2,8 @@
 
 import argparse
 import sys
+from typing import Optional
+from typing import Sequence
 
 BLACKLIST = [
     b'BEGIN RSA PRIVATE KEY',
@@ -15,7 +17,7 @@
 ]
 
 
-def detect_private_key(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*', help='Filenames to check')
     args = parser.parse_args(argv)
@@ -37,4 +39,4 @@
 
 
 if __name__ == '__main__':
-    sys.exit(detect_private_key())
+    sys.exit(main())
diff --git a/pre_commit_hooks/end_of_file_fixer.py b/pre_commit_hooks/end_of_file_fixer.py
index 5ab1b7b..4e77c94 100644
--- a/pre_commit_hooks/end_of_file_fixer.py
+++ b/pre_commit_hooks/end_of_file_fixer.py
@@ -4,9 +4,12 @@
 import argparse
 import os
 import sys
+from typing import IO
+from typing import Optional
+from typing import Sequence
 
 
-def fix_file(file_obj):
+def fix_file(file_obj):  # type: (IO[bytes]) -> int
     # Test for newline at end of file
     # Empty files will throw IOError here
     try:
@@ -49,7 +52,7 @@
     return 0
 
 
-def end_of_file_fixer(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*', help='Filenames to fix')
     args = parser.parse_args(argv)
@@ -68,4 +71,4 @@
 
 
 if __name__ == '__main__':
-    sys.exit(end_of_file_fixer())
+    sys.exit(main())
diff --git a/pre_commit_hooks/file_contents_sorter.py b/pre_commit_hooks/file_contents_sorter.py
index fe7f7ee..6f13c98 100644
--- a/pre_commit_hooks/file_contents_sorter.py
+++ b/pre_commit_hooks/file_contents_sorter.py
@@ -12,12 +12,15 @@
 from __future__ import print_function
 
 import argparse
+from typing import IO
+from typing import Optional
+from typing import Sequence
 
 PASS = 0
 FAIL = 1
 
 
-def sort_file_contents(f):
+def sort_file_contents(f):  # type: (IO[bytes]) -> int
     before = list(f)
     after = sorted([line.strip(b'\n\r') for line in before if line.strip()])
 
@@ -33,7 +36,7 @@
         return FAIL
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='+', help='Files to sort')
     args = parser.parse_args(argv)
diff --git a/pre_commit_hooks/fix_encoding_pragma.py b/pre_commit_hooks/fix_encoding_pragma.py
index 3bf234e..b0b5c8e 100644
--- a/pre_commit_hooks/fix_encoding_pragma.py
+++ b/pre_commit_hooks/fix_encoding_pragma.py
@@ -4,11 +4,15 @@
 
 import argparse
 import collections
+from typing import IO
+from typing import Optional
+from typing import Sequence
+from typing import Union
 
 DEFAULT_PRAGMA = b'# -*- coding: utf-8 -*-\n'
 
 
-def has_coding(line):
+def has_coding(line):  # type: (bytes) -> bool
     if not line.strip():
         return False
     return (
@@ -33,15 +37,16 @@
     __slots__ = ()
 
     @property
-    def has_any_pragma(self):
+    def has_any_pragma(self):  # type: () -> bool
         return self.pragma_status is not False
 
-    def is_expected_pragma(self, remove):
+    def is_expected_pragma(self, remove):  # type: (bool) -> bool
         expected_pragma_status = not remove
         return self.pragma_status is expected_pragma_status
 
 
 def _get_expected_contents(first_line, second_line, rest, expected_pragma):
+    # type: (bytes, bytes, bytes, bytes) -> ExpectedContents
     if first_line.startswith(b'#!'):
         shebang = first_line
         potential_coding = second_line
@@ -51,7 +56,7 @@
         rest = second_line + rest
 
     if potential_coding == expected_pragma:
-        pragma_status = True
+        pragma_status = True  # type: Optional[bool]
     elif has_coding(potential_coding):
         pragma_status = None
     else:
@@ -64,6 +69,7 @@
 
 
 def fix_encoding_pragma(f, remove=False, expected_pragma=DEFAULT_PRAGMA):
+    # type: (IO[bytes], bool, bytes) -> int
     expected = _get_expected_contents(
         f.readline(), f.readline(), f.read(), expected_pragma,
     )
@@ -93,17 +99,17 @@
     return 1
 
 
-def _normalize_pragma(pragma):
+def _normalize_pragma(pragma):  # type: (Union[bytes, str]) -> bytes
     if not isinstance(pragma, bytes):
         pragma = pragma.encode('UTF-8')
     return pragma.rstrip() + b'\n'
 
 
-def _to_disp(pragma):
+def _to_disp(pragma):  # type: (bytes) -> str
     return pragma.decode().rstrip()
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser('Fixes the encoding pragma of python files')
     parser.add_argument('filenames', nargs='*', help='Filenames to fix')
     parser.add_argument(
diff --git a/pre_commit_hooks/forbid_new_submodules.py b/pre_commit_hooks/forbid_new_submodules.py
index c9464cf..bdbd6f7 100644
--- a/pre_commit_hooks/forbid_new_submodules.py
+++ b/pre_commit_hooks/forbid_new_submodules.py
@@ -2,10 +2,13 @@
 from __future__ import print_function
 from __future__ import unicode_literals
 
+from typing import Optional
+from typing import Sequence
+
 from pre_commit_hooks.util import cmd_output
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     # `argv` is ignored, pre-commit will send us a list of files that we
     # don't care about
     added_diff = cmd_output(
diff --git a/pre_commit_hooks/mixed_line_ending.py b/pre_commit_hooks/mixed_line_ending.py
index e35a65c..90aef03 100644
--- a/pre_commit_hooks/mixed_line_ending.py
+++ b/pre_commit_hooks/mixed_line_ending.py
@@ -4,6 +4,9 @@
 
 import argparse
 import collections
+from typing import Dict
+from typing import Optional
+from typing import Sequence
 
 
 CRLF = b'\r\n'
@@ -14,7 +17,7 @@
 FIX_TO_LINE_ENDING = {'cr': CR, 'crlf': CRLF, 'lf': LF}
 
 
-def _fix(filename, contents, ending):
+def _fix(filename, contents, ending):  # type: (str, bytes, bytes) -> None
     new_contents = b''.join(
         line.rstrip(b'\r\n') + ending for line in contents.splitlines(True)
     )
@@ -22,11 +25,11 @@
         f.write(new_contents)
 
 
-def fix_filename(filename, fix):
+def fix_filename(filename, fix):  # type: (str, str) -> int
     with open(filename, 'rb') as f:
         contents = f.read()
 
-    counts = collections.defaultdict(int)
+    counts = collections.defaultdict(int)  # type: Dict[bytes, int]
 
     for line in contents.splitlines(True):
         for ending in ALL_ENDINGS:
@@ -63,7 +66,7 @@
         return other_endings
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument(
         '-f', '--fix',
diff --git a/pre_commit_hooks/no_commit_to_branch.py b/pre_commit_hooks/no_commit_to_branch.py
index fdd146b..6b68c91 100644
--- a/pre_commit_hooks/no_commit_to_branch.py
+++ b/pre_commit_hooks/no_commit_to_branch.py
@@ -1,12 +1,15 @@
 from __future__ import print_function
 
 import argparse
+from typing import Optional
+from typing import Sequence
+from typing import Set
 
 from pre_commit_hooks.util import CalledProcessError
 from pre_commit_hooks.util import cmd_output
 
 
-def is_on_branch(protected):
+def is_on_branch(protected):  # type: (Set[str]) -> bool
     try:
         branch = cmd_output('git', 'symbolic-ref', 'HEAD')
     except CalledProcessError:
@@ -15,7 +18,7 @@
     return '/'.join(chunks[2:]) in protected
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument(
         '-b', '--branch', action='append',
diff --git a/pre_commit_hooks/pretty_format_json.py b/pre_commit_hooks/pretty_format_json.py
index 363037e..de7f8d7 100644
--- a/pre_commit_hooks/pretty_format_json.py
+++ b/pre_commit_hooks/pretty_format_json.py
@@ -5,12 +5,20 @@
 import json
 import sys
 from collections import OrderedDict
+from typing import List
+from typing import Mapping
+from typing import Optional
+from typing import Sequence
+from typing import Tuple
+from typing import Union
 
 from six import text_type
 
 
-def _get_pretty_format(contents, indent, ensure_ascii=True, sort_keys=True, top_keys=[]):
+def _get_pretty_format(contents, indent, ensure_ascii=True, sort_keys=True, top_keys=()):
+    # type: (str, str, bool, bool, Sequence[str]) -> str
     def pairs_first(pairs):
+        # type: (Sequence[Tuple[str, str]]) -> Mapping[str, str]
         before = [pair for pair in pairs if pair[0] in top_keys]
         before = sorted(before, key=lambda x: top_keys.index(x[0]))
         after = [pair for pair in pairs if pair[0] not in top_keys]
@@ -27,13 +35,13 @@
     return text_type(json_pretty) + '\n'
 
 
-def _autofix(filename, new_contents):
+def _autofix(filename, new_contents):  # type: (str, str) -> None
     print('Fixing file {}'.format(filename))
     with io.open(filename, 'w', encoding='UTF-8') as f:
         f.write(new_contents)
 
 
-def parse_num_to_int(s):
+def parse_num_to_int(s):  # type: (str) -> Union[int, str]
     """Convert string numbers to int, leaving strings as is."""
     try:
         return int(s)
@@ -41,11 +49,11 @@
         return s
 
 
-def parse_topkeys(s):
+def parse_topkeys(s):  # type: (str) -> List[str]
     return s.split(',')
 
 
-def pretty_format_json(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument(
         '--autofix',
@@ -117,4 +125,4 @@
 
 
 if __name__ == '__main__':
-    sys.exit(pretty_format_json())
+    sys.exit(main())
diff --git a/pre_commit_hooks/requirements_txt_fixer.py b/pre_commit_hooks/requirements_txt_fixer.py
index 6dcf8d0..3f85a17 100644
--- a/pre_commit_hooks/requirements_txt_fixer.py
+++ b/pre_commit_hooks/requirements_txt_fixer.py
@@ -1,6 +1,10 @@
 from __future__ import print_function
 
 import argparse
+from typing import IO
+from typing import List
+from typing import Optional
+from typing import Sequence
 
 
 PASS = 0
@@ -9,21 +13,23 @@
 
 class Requirement(object):
 
-    def __init__(self):
+    def __init__(self):  # type: () -> None
         super(Requirement, self).__init__()
-        self.value = None
-        self.comments = []
+        self.value = None  # type: Optional[bytes]
+        self.comments = []   # type: List[bytes]
 
     @property
-    def name(self):
+    def name(self):  # type: () -> bytes
+        assert self.value is not None, self.value
         if self.value.startswith(b'-e '):
             return self.value.lower().partition(b'=')[-1]
 
         return self.value.lower().partition(b'==')[0]
 
-    def __lt__(self, requirement):
+    def __lt__(self, requirement):  # type: (Requirement) -> int
         # \n means top of file comment, so always return True,
         # otherwise just do a string comparison with value.
+        assert self.value is not None, self.value
         if self.value == b'\n':
             return True
         elif requirement.value == b'\n':
@@ -32,10 +38,10 @@
             return self.name < requirement.name
 
 
-def fix_requirements(f):
-    requirements = []
+def fix_requirements(f):  # type: (IO[bytes]) -> int
+    requirements = []  # type: List[Requirement]
     before = tuple(f)
-    after = []
+    after = []  # type: List[bytes]
 
     before_string = b''.join(before)
 
@@ -46,6 +52,7 @@
     for line in before:
         # If the most recent requirement object has a value, then it's
         # time to start building the next requirement object.
+
         if not len(requirements) or requirements[-1].value is not None:
             requirements.append(Requirement())
 
@@ -78,6 +85,7 @@
 
     for requirement in sorted(requirements):
         after.extend(requirement.comments)
+        assert requirement.value, requirement.value
         after.append(requirement.value)
     after.extend(rest)
 
@@ -92,7 +100,7 @@
         return FAIL
 
 
-def fix_requirements_txt(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*', help='Filenames to fix')
     args = parser.parse_args(argv)
@@ -109,3 +117,7 @@
             retv |= ret_for_file
 
     return retv
+
+
+if __name__ == '__main__':
+    exit(main())
diff --git a/pre_commit_hooks/sort_simple_yaml.py b/pre_commit_hooks/sort_simple_yaml.py
index 7afae91..3c8ef16 100755
--- a/pre_commit_hooks/sort_simple_yaml.py
+++ b/pre_commit_hooks/sort_simple_yaml.py
@@ -21,12 +21,15 @@
 from __future__ import print_function
 
 import argparse
+from typing import List
+from typing import Optional
+from typing import Sequence
 
 
 QUOTES = ["'", '"']
 
 
-def sort(lines):
+def sort(lines):  # type: (List[str]) -> List[str]
     """Sort a YAML file in alphabetical order, keeping blocks together.
 
     :param lines: array of strings (without newlines)
@@ -44,7 +47,7 @@
     return new_lines
 
 
-def parse_block(lines, header=False):
+def parse_block(lines, header=False):  # type: (List[str], bool) -> List[str]
     """Parse and return a single block, popping off the start of `lines`.
 
     If parsing a header block, we stop after we reach a line that is not a
@@ -60,7 +63,7 @@
     return block_lines
 
 
-def parse_blocks(lines):
+def parse_blocks(lines):  # type: (List[str]) -> List[List[str]]
     """Parse and return all possible blocks, popping off the start of `lines`.
 
     :param lines: list of lines
@@ -77,7 +80,7 @@
     return blocks
 
 
-def first_key(lines):
+def first_key(lines):  # type: (List[str]) -> str
     """Returns a string representing the sort key of a block.
 
     The sort key is the first YAML key we encounter, ignoring comments, and
@@ -95,9 +98,11 @@
         if any(line.startswith(quote) for quote in QUOTES):
             return line[1:]
         return line
+    else:
+        return ''  # not actually reached in reality
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*', help='Filenames to fix')
     args = parser.parse_args(argv)
diff --git a/pre_commit_hooks/string_fixer.py b/pre_commit_hooks/string_fixer.py
index c432682..a5ea1ea 100644
--- a/pre_commit_hooks/string_fixer.py
+++ b/pre_commit_hooks/string_fixer.py
@@ -4,34 +4,39 @@
 
 import argparse
 import io
+import re
 import tokenize
+from typing import List
+from typing import Optional
+from typing import Sequence
+
+START_QUOTE_RE = re.compile('^[a-zA-Z]*"')
 
 
-double_quote_starts = tuple(s for s in tokenize.single_quoted if '"' in s)
-
-
-def handle_match(token_text):
+def handle_match(token_text):  # type: (str) -> str
     if '"""' in token_text or "'''" in token_text:
         return token_text
 
-    for double_quote_start in double_quote_starts:
-        if token_text.startswith(double_quote_start):
-            meat = token_text[len(double_quote_start):-1]
-            if '"' in meat or "'" in meat:
-                break
-            return double_quote_start.replace('"', "'") + meat + "'"
-    return token_text
+    match = START_QUOTE_RE.match(token_text)
+    if match is not None:
+        meat = token_text[match.end():-1]
+        if '"' in meat or "'" in meat:
+            return token_text
+        else:
+            return match.group().replace('"', "'") + meat + "'"
+    else:
+        return token_text
 
 
-def get_line_offsets_by_line_no(src):
+def get_line_offsets_by_line_no(src):  # type: (str) -> List[int]
     # Padded so we can index with line number
-    offsets = [None, 0]
+    offsets = [-1, 0]
     for line in src.splitlines():
         offsets.append(offsets[-1] + len(line) + 1)
     return offsets
 
 
-def fix_strings(filename):
+def fix_strings(filename):  # type: (str) -> int
     with io.open(filename, encoding='UTF-8') as f:
         contents = f.read()
     line_offsets = get_line_offsets_by_line_no(contents)
@@ -60,7 +65,7 @@
         return 0
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*', help='Filenames to fix')
     args = parser.parse_args(argv)
@@ -74,3 +79,7 @@
         retv |= return_value
 
     return retv
+
+
+if __name__ == '__main__':
+    exit(main())
diff --git a/pre_commit_hooks/tests_should_end_in_test.py b/pre_commit_hooks/tests_should_end_in_test.py
index 9bea20d..7a1e7c0 100644
--- a/pre_commit_hooks/tests_should_end_in_test.py
+++ b/pre_commit_hooks/tests_should_end_in_test.py
@@ -1,12 +1,14 @@
 from __future__ import print_function
 
 import argparse
+import os.path
 import re
 import sys
-from os.path import basename
+from typing import Optional
+from typing import Sequence
 
 
-def validate_files(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument('filenames', nargs='*')
     parser.add_argument(
@@ -18,7 +20,7 @@
     retcode = 0
     test_name_pattern = 'test.*.py' if args.django else '.*_test.py'
     for filename in args.filenames:
-        base = basename(filename)
+        base = os.path.basename(filename)
         if (
                 not re.match(test_name_pattern, base) and
                 not base == '__init__.py' and
@@ -35,4 +37,4 @@
 
 
 if __name__ == '__main__':
-    sys.exit(validate_files())
+    sys.exit(main())
diff --git a/pre_commit_hooks/trailing_whitespace_fixer.py b/pre_commit_hooks/trailing_whitespace_fixer.py
index 1b54fbd..4fe7975 100644
--- a/pre_commit_hooks/trailing_whitespace_fixer.py
+++ b/pre_commit_hooks/trailing_whitespace_fixer.py
@@ -3,9 +3,11 @@
 import argparse
 import os
 import sys
+from typing import Optional
+from typing import Sequence
 
 
-def _fix_file(filename, is_markdown):
+def _fix_file(filename, is_markdown):  # type: (str, bool) -> bool
     with open(filename, mode='rb') as file_processed:
         lines = file_processed.readlines()
     newlines = [_process_line(line, is_markdown) for line in lines]
@@ -18,7 +20,7 @@
         return False
 
 
-def _process_line(line, is_markdown):
+def _process_line(line, is_markdown):  # type: (bytes, bool) -> bytes
     if line[-2:] == b'\r\n':
         eol = b'\r\n'
     elif line[-1:] == b'\n':
@@ -31,7 +33,7 @@
     return line.rstrip() + eol
 
 
-def main(argv=None):
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
     parser = argparse.ArgumentParser()
     parser.add_argument(
         '--no-markdown-linebreak-ext',
diff --git a/pre_commit_hooks/util.py b/pre_commit_hooks/util.py
index 269b553..5d1d11b 100644
--- a/pre_commit_hooks/util.py
+++ b/pre_commit_hooks/util.py
@@ -3,23 +3,25 @@
 from __future__ import unicode_literals
 
 import subprocess
+from typing import Any
+from typing import Set
 
 
 class CalledProcessError(RuntimeError):
     pass
 
 
-def added_files():
+def added_files():  # type: () -> Set[str]
     return set(cmd_output(
         'git', 'diff', '--staged', '--name-only', '--diff-filter=A',
     ).splitlines())
 
 
-def cmd_output(*cmd, **kwargs):
+def cmd_output(*cmd, **kwargs):  # type: (*str, **Any) -> str
     retcode = kwargs.pop('retcode', 0)
-    popen_kwargs = {'stdout': subprocess.PIPE, 'stderr': subprocess.PIPE}
-    popen_kwargs.update(kwargs)
-    proc = subprocess.Popen(cmd, **popen_kwargs)
+    kwargs.setdefault('stdout', subprocess.PIPE)
+    kwargs.setdefault('stderr', subprocess.PIPE)
+    proc = subprocess.Popen(cmd, **kwargs)
     stdout, stderr = proc.communicate()
     stdout = stdout.decode('UTF-8')
     if stderr is not None:
diff --git a/setup.py b/setup.py
index 84892a7..756500b 100644
--- a/setup.py
+++ b/setup.py
@@ -28,35 +28,36 @@
         'ruamel.yaml>=0.15',
         'six',
     ],
+    extras_require={':python_version<"3.5"': ['typing']},
     entry_points={
         'console_scripts': [
             'autopep8-wrapper = pre_commit_hooks.autopep8_wrapper:main',
             'check-added-large-files = pre_commit_hooks.check_added_large_files:main',
-            'check-ast = pre_commit_hooks.check_ast:check_ast',
+            'check-ast = pre_commit_hooks.check_ast:main',
             'check-builtin-literals = pre_commit_hooks.check_builtin_literals:main',
             'check-byte-order-marker = pre_commit_hooks.check_byte_order_marker:main',
             'check-case-conflict = pre_commit_hooks.check_case_conflict:main',
             'check-docstring-first = pre_commit_hooks.check_docstring_first:main',
             'check-executables-have-shebangs = pre_commit_hooks.check_executables_have_shebangs:main',
-            'check-json = pre_commit_hooks.check_json:check_json',
-            'check-merge-conflict = pre_commit_hooks.check_merge_conflict:detect_merge_conflict',
-            'check-symlinks = pre_commit_hooks.check_symlinks:check_symlinks',
+            'check-json = pre_commit_hooks.check_json:main',
+            'check-merge-conflict = pre_commit_hooks.check_merge_conflict:main',
+            'check-symlinks = pre_commit_hooks.check_symlinks:main',
             'check-vcs-permalinks = pre_commit_hooks.check_vcs_permalinks:main',
-            'check-xml = pre_commit_hooks.check_xml:check_xml',
-            'check-yaml = pre_commit_hooks.check_yaml:check_yaml',
+            'check-xml = pre_commit_hooks.check_xml:main',
+            'check-yaml = pre_commit_hooks.check_yaml:main',
             'debug-statement-hook = pre_commit_hooks.debug_statement_hook:main',
             'detect-aws-credentials = pre_commit_hooks.detect_aws_credentials:main',
-            'detect-private-key = pre_commit_hooks.detect_private_key:detect_private_key',
+            'detect-private-key = pre_commit_hooks.detect_private_key:main',
             'double-quote-string-fixer = pre_commit_hooks.string_fixer:main',
-            'end-of-file-fixer = pre_commit_hooks.end_of_file_fixer:end_of_file_fixer',
+            'end-of-file-fixer = pre_commit_hooks.end_of_file_fixer:main',
             'file-contents-sorter = pre_commit_hooks.file_contents_sorter:main',
             'fix-encoding-pragma = pre_commit_hooks.fix_encoding_pragma:main',
             'forbid-new-submodules = pre_commit_hooks.forbid_new_submodules:main',
             'mixed-line-ending = pre_commit_hooks.mixed_line_ending:main',
-            'name-tests-test = pre_commit_hooks.tests_should_end_in_test:validate_files',
+            'name-tests-test = pre_commit_hooks.tests_should_end_in_test:main',
             'no-commit-to-branch = pre_commit_hooks.no_commit_to_branch:main',
-            'pretty-format-json = pre_commit_hooks.pretty_format_json:pretty_format_json',
-            'requirements-txt-fixer = pre_commit_hooks.requirements_txt_fixer:fix_requirements_txt',
+            'pretty-format-json = pre_commit_hooks.pretty_format_json:main',
+            'requirements-txt-fixer = pre_commit_hooks.requirements_txt_fixer:main',
             'sort-simple-yaml = pre_commit_hooks.sort_simple_yaml:main',
             'trailing-whitespace-fixer = pre_commit_hooks.trailing_whitespace_fixer:main',
         ],
diff --git a/testing/resources/bad_json_latin1.nonjson b/testing/resources/bad_json_latin1.nonjson
old mode 100755
new mode 100644
diff --git a/testing/resources/builtin_constructors.py b/testing/resources/builtin_constructors.py
deleted file mode 100644
index 174a9e8..0000000
--- a/testing/resources/builtin_constructors.py
+++ /dev/null
@@ -1,17 +0,0 @@
-from six.moves import builtins
-
-c1 = complex()
-d1 = dict()
-f1 = float()
-i1 = int()
-l1 = list()
-s1 = str()
-t1 = tuple()
-
-c2 = builtins.complex()
-d2 = builtins.dict()
-f2 = builtins.float()
-i2 = builtins.int()
-l2 = builtins.list()
-s2 = builtins.str()
-t2 = builtins.tuple()
diff --git a/testing/resources/builtin_literals.py b/testing/resources/builtin_literals.py
deleted file mode 100644
index 8513b70..0000000
--- a/testing/resources/builtin_literals.py
+++ /dev/null
@@ -1,7 +0,0 @@
-c1 = 0j
-d1 = {}
-f1 = 0.0
-i1 = 0
-l1 = []
-s1 = ''
-t1 = ()
diff --git a/tests/check_ast_test.py b/tests/check_ast_test.py
index 64916ba..c16f5fc 100644
--- a/tests/check_ast_test.py
+++ b/tests/check_ast_test.py
@@ -1,15 +1,15 @@
 from __future__ import absolute_import
 from __future__ import unicode_literals
 
-from pre_commit_hooks.check_ast import check_ast
+from pre_commit_hooks.check_ast import main
 from testing.util import get_resource_path
 
 
 def test_failing_file():
-    ret = check_ast([get_resource_path('cannot_parse_ast.notpy')])
+    ret = main([get_resource_path('cannot_parse_ast.notpy')])
     assert ret == 1
 
 
 def test_passing_file():
-    ret = check_ast([__file__])
+    ret = main([__file__])
     assert ret == 0
diff --git a/tests/check_builtin_literals_test.py b/tests/check_builtin_literals_test.py
index 86b79e3..d4ac30f 100644
--- a/tests/check_builtin_literals_test.py
+++ b/tests/check_builtin_literals_test.py
@@ -5,7 +5,35 @@
 from pre_commit_hooks.check_builtin_literals import BuiltinTypeCall
 from pre_commit_hooks.check_builtin_literals import BuiltinTypeVisitor
 from pre_commit_hooks.check_builtin_literals import main
-from testing.util import get_resource_path
+
+BUILTIN_CONSTRUCTORS = '''\
+from six.moves import builtins
+
+c1 = complex()
+d1 = dict()
+f1 = float()
+i1 = int()
+l1 = list()
+s1 = str()
+t1 = tuple()
+
+c2 = builtins.complex()
+d2 = builtins.dict()
+f2 = builtins.float()
+i2 = builtins.int()
+l2 = builtins.list()
+s2 = builtins.str()
+t2 = builtins.tuple()
+'''
+BUILTIN_LITERALS = '''\
+c1 = 0j
+d1 = {}
+f1 = 0.0
+i1 = 0
+l1 = []
+s1 = ''
+t1 = ()
+'''
 
 
 @pytest.fixture
@@ -94,24 +122,26 @@
 
 def test_ignore_constructors():
     visitor = BuiltinTypeVisitor(ignore=('complex', 'dict', 'float', 'int', 'list', 'str', 'tuple'))
-    with open(get_resource_path('builtin_constructors.py'), 'rb') as f:
-        visitor.visit(ast.parse(f.read(), 'builtin_constructors.py'))
+    visitor.visit(ast.parse(BUILTIN_CONSTRUCTORS))
     assert visitor.builtin_type_calls == []
 
 
-def test_failing_file():
-    rc = main([get_resource_path('builtin_constructors.py')])
+def test_failing_file(tmpdir):
+    f = tmpdir.join('f.py')
+    f.write(BUILTIN_CONSTRUCTORS)
+    rc = main([f.strpath])
     assert rc == 1
 
 
-def test_passing_file():
-    rc = main([get_resource_path('builtin_literals.py')])
+def test_passing_file(tmpdir):
+    f = tmpdir.join('f.py')
+    f.write(BUILTIN_LITERALS)
+    rc = main([f.strpath])
     assert rc == 0
 
 
-def test_failing_file_ignore_all():
-    rc = main([
-        '--ignore=complex,dict,float,int,list,str,tuple',
-        get_resource_path('builtin_constructors.py'),
-    ])
+def test_failing_file_ignore_all(tmpdir):
+    f = tmpdir.join('f.py')
+    f.write(BUILTIN_CONSTRUCTORS)
+    rc = main(['--ignore=complex,dict,float,int,list,str,tuple', f.strpath])
     assert rc == 0
diff --git a/tests/check_json_test.py b/tests/check_json_test.py
index 6ba26c1..6654ed1 100644
--- a/tests/check_json_test.py
+++ b/tests/check_json_test.py
@@ -1,6 +1,6 @@
 import pytest
 
-from pre_commit_hooks.check_json import check_json
+from pre_commit_hooks.check_json import main
 from testing.util import get_resource_path
 
 
@@ -11,8 +11,8 @@
         ('ok_json.json', 0),
     ),
 )
-def test_check_json(capsys, filename, expected_retval):
-    ret = check_json([get_resource_path(filename)])
+def test_main(capsys, filename, expected_retval):
+    ret = main([get_resource_path(filename)])
     assert ret == expected_retval
     if expected_retval == 1:
         stdout, _ = capsys.readouterr()
diff --git a/tests/check_merge_conflict_test.py b/tests/check_merge_conflict_test.py
index b04c70e..50e389c 100644
--- a/tests/check_merge_conflict_test.py
+++ b/tests/check_merge_conflict_test.py
@@ -6,7 +6,7 @@
 
 import pytest
 
-from pre_commit_hooks.check_merge_conflict import detect_merge_conflict
+from pre_commit_hooks.check_merge_conflict import main
 from pre_commit_hooks.util import cmd_output
 from testing.util import get_resource_path
 
@@ -102,7 +102,7 @@
 
 @pytest.mark.usefixtures('f1_is_a_conflict_file')
 def test_merge_conflicts_git():
-    assert detect_merge_conflict(['f1']) == 1
+    assert main(['f1']) == 1
 
 
 @pytest.mark.parametrize(
@@ -110,7 +110,7 @@
 )
 def test_merge_conflicts_failing(contents, repository_pending_merge):
     repository_pending_merge.join('f2').write_binary(contents)
-    assert detect_merge_conflict(['f2']) == 1
+    assert main(['f2']) == 1
 
 
 @pytest.mark.parametrize(
@@ -118,22 +118,22 @@
 )
 def test_merge_conflicts_ok(contents, f1_is_a_conflict_file):
     f1_is_a_conflict_file.join('f1').write_binary(contents)
-    assert detect_merge_conflict(['f1']) == 0
+    assert main(['f1']) == 0
 
 
 @pytest.mark.usefixtures('f1_is_a_conflict_file')
 def test_ignores_binary_files():
     shutil.copy(get_resource_path('img1.jpg'), 'f1')
-    assert detect_merge_conflict(['f1']) == 0
+    assert main(['f1']) == 0
 
 
 def test_does_not_care_when_not_in_a_merge(tmpdir):
     f = tmpdir.join('README.md')
     f.write_binary(b'problem\n=======\n')
-    assert detect_merge_conflict([str(f.realpath())]) == 0
+    assert main([str(f.realpath())]) == 0
 
 
 def test_care_when_assumed_merge(tmpdir):
     f = tmpdir.join('README.md')
     f.write_binary(b'problem\n=======\n')
-    assert detect_merge_conflict([str(f.realpath()), '--assume-in-merge']) == 1
+    assert main([str(f.realpath()), '--assume-in-merge']) == 1
diff --git a/tests/check_symlinks_test.py b/tests/check_symlinks_test.py
index 0414df5..ecbc7ae 100644
--- a/tests/check_symlinks_test.py
+++ b/tests/check_symlinks_test.py
@@ -2,7 +2,7 @@
 
 import pytest
 
-from pre_commit_hooks.check_symlinks import check_symlinks
+from pre_commit_hooks.check_symlinks import main
 
 
 xfail_symlink = pytest.mark.xfail(os.name == 'nt', reason='No symlink support')
@@ -12,12 +12,12 @@
 @pytest.mark.parametrize(
     ('dest', 'expected'), (('exists', 0), ('does-not-exist', 1)),
 )
-def test_check_symlinks(tmpdir, dest, expected):  # pragma: no cover (symlinks)
+def test_main(tmpdir, dest, expected):  # pragma: no cover (symlinks)
     tmpdir.join('exists').ensure()
     symlink = tmpdir.join('symlink')
     symlink.mksymlinkto(tmpdir.join(dest))
-    assert check_symlinks((symlink.strpath,)) == expected
+    assert main((symlink.strpath,)) == expected
 
 
-def test_check_symlinks_normal_file(tmpdir):
-    assert check_symlinks((tmpdir.join('f').ensure().strpath,)) == 0
+def test_main_normal_file(tmpdir):
+    assert main((tmpdir.join('f').ensure().strpath,)) == 0
diff --git a/tests/check_xml_test.py b/tests/check_xml_test.py
index 84e365d..357bad6 100644
--- a/tests/check_xml_test.py
+++ b/tests/check_xml_test.py
@@ -1,6 +1,6 @@
 import pytest
 
-from pre_commit_hooks.check_xml import check_xml
+from pre_commit_hooks.check_xml import main
 from testing.util import get_resource_path
 
 
@@ -10,6 +10,6 @@
         ('ok_xml.xml', 0),
     ),
 )
-def test_check_xml(filename, expected_retval):
-    ret = check_xml([get_resource_path(filename)])
+def test_main(filename, expected_retval):
+    ret = main([get_resource_path(filename)])
     assert ret == expected_retval
diff --git a/tests/check_yaml_test.py b/tests/check_yaml_test.py
index aa357f1..d267150 100644
--- a/tests/check_yaml_test.py
+++ b/tests/check_yaml_test.py
@@ -3,7 +3,7 @@
 
 import pytest
 
-from pre_commit_hooks.check_yaml import check_yaml
+from pre_commit_hooks.check_yaml import main
 from testing.util import get_resource_path
 
 
@@ -13,29 +13,29 @@
         ('ok_yaml.yaml', 0),
     ),
 )
-def test_check_yaml(filename, expected_retval):
-    ret = check_yaml([get_resource_path(filename)])
+def test_main(filename, expected_retval):
+    ret = main([get_resource_path(filename)])
     assert ret == expected_retval
 
 
-def test_check_yaml_allow_multiple_documents(tmpdir):
+def test_main_allow_multiple_documents(tmpdir):
     f = tmpdir.join('test.yaml')
     f.write('---\nfoo\n---\nbar\n')
 
     # should fail without the setting
-    assert check_yaml((f.strpath,))
+    assert main((f.strpath,))
 
     # should pass when we allow multiple documents
-    assert not check_yaml(('--allow-multiple-documents', f.strpath))
+    assert not main(('--allow-multiple-documents', f.strpath))
 
 
 def test_fails_even_with_allow_multiple_documents(tmpdir):
     f = tmpdir.join('test.yaml')
     f.write('[')
-    assert check_yaml(('--allow-multiple-documents', f.strpath))
+    assert main(('--allow-multiple-documents', f.strpath))
 
 
-def test_check_yaml_unsafe(tmpdir):
+def test_main_unsafe(tmpdir):
     f = tmpdir.join('test.yaml')
     f.write(
         'some_foo: !vault |\n'
@@ -43,12 +43,12 @@
         '    deadbeefdeadbeefdeadbeef\n',
     )
     # should fail "safe" check
-    assert check_yaml((f.strpath,))
+    assert main((f.strpath,))
     # should pass when we allow unsafe documents
-    assert not check_yaml(('--unsafe', f.strpath))
+    assert not main(('--unsafe', f.strpath))
 
 
-def test_check_yaml_unsafe_still_fails_on_syntax_errors(tmpdir):
+def test_main_unsafe_still_fails_on_syntax_errors(tmpdir):
     f = tmpdir.join('test.yaml')
     f.write('[')
-    assert check_yaml(('--unsafe', f.strpath))
+    assert main(('--unsafe', f.strpath))
diff --git a/tests/detect_private_key_test.py b/tests/detect_private_key_test.py
index fdd63a2..9266f2b 100644
--- a/tests/detect_private_key_test.py
+++ b/tests/detect_private_key_test.py
@@ -1,6 +1,6 @@
 import pytest
 
-from pre_commit_hooks.detect_private_key import detect_private_key
+from pre_commit_hooks.detect_private_key import main
 
 # Input, expected return value
 TESTS = (
@@ -18,7 +18,7 @@
 
 
 @pytest.mark.parametrize(('input_s', 'expected_retval'), TESTS)
-def test_detect_private_key(input_s, expected_retval, tmpdir):
+def test_main(input_s, expected_retval, tmpdir):
     path = tmpdir.join('file.txt')
     path.write_binary(input_s)
-    assert detect_private_key([path.strpath]) == expected_retval
+    assert main([path.strpath]) == expected_retval
diff --git a/tests/end_of_file_fixer_test.py b/tests/end_of_file_fixer_test.py
index f8710af..7f644e7 100644
--- a/tests/end_of_file_fixer_test.py
+++ b/tests/end_of_file_fixer_test.py
@@ -2,8 +2,8 @@
 
 import pytest
 
-from pre_commit_hooks.end_of_file_fixer import end_of_file_fixer
 from pre_commit_hooks.end_of_file_fixer import fix_file
+from pre_commit_hooks.end_of_file_fixer import main
 
 
 # Input, expected return value, expected output
@@ -35,7 +35,7 @@
     path = tmpdir.join('file.txt')
     path.write_binary(input_s)
 
-    ret = end_of_file_fixer([path.strpath])
+    ret = main([path.strpath])
     file_output = path.read_binary()
 
     assert file_output == output
diff --git a/tests/no_commit_to_branch_test.py b/tests/no_commit_to_branch_test.py
index c275bf7..e978ba2 100644
--- a/tests/no_commit_to_branch_test.py
+++ b/tests/no_commit_to_branch_test.py
@@ -11,24 +11,24 @@
 def test_other_branch(temp_git_dir):
     with temp_git_dir.as_cwd():
         cmd_output('git', 'checkout', '-b', 'anotherbranch')
-        assert is_on_branch(('master',)) is False
+        assert is_on_branch({'master'}) is False
 
 
 def test_multi_branch(temp_git_dir):
     with temp_git_dir.as_cwd():
         cmd_output('git', 'checkout', '-b', 'another/branch')
-        assert is_on_branch(('master',)) is False
+        assert is_on_branch({'master'}) is False
 
 
 def test_multi_branch_fail(temp_git_dir):
     with temp_git_dir.as_cwd():
         cmd_output('git', 'checkout', '-b', 'another/branch')
-        assert is_on_branch(('another/branch',)) is True
+        assert is_on_branch({'another/branch'}) is True
 
 
 def test_master_branch(temp_git_dir):
     with temp_git_dir.as_cwd():
-        assert is_on_branch(('master',)) is True
+        assert is_on_branch({'master'}) is True
 
 
 def test_main_branch_call(temp_git_dir):
diff --git a/tests/pretty_format_json_test.py b/tests/pretty_format_json_test.py
index 7ce7e16..8d82d74 100644
--- a/tests/pretty_format_json_test.py
+++ b/tests/pretty_format_json_test.py
@@ -3,8 +3,8 @@
 import pytest
 from six import PY2
 
+from pre_commit_hooks.pretty_format_json import main
 from pre_commit_hooks.pretty_format_json import parse_num_to_int
-from pre_commit_hooks.pretty_format_json import pretty_format_json
 from testing.util import get_resource_path
 
 
@@ -23,8 +23,8 @@
         ('pretty_formatted_json.json', 0),
     ),
 )
-def test_pretty_format_json(filename, expected_retval):
-    ret = pretty_format_json([get_resource_path(filename)])
+def test_main(filename, expected_retval):
+    ret = main([get_resource_path(filename)])
     assert ret == expected_retval
 
 
@@ -36,8 +36,8 @@
         ('pretty_formatted_json.json', 0),
     ),
 )
-def test_unsorted_pretty_format_json(filename, expected_retval):
-    ret = pretty_format_json(['--no-sort-keys', get_resource_path(filename)])
+def test_unsorted_main(filename, expected_retval):
+    ret = main(['--no-sort-keys', get_resource_path(filename)])
     assert ret == expected_retval
 
 
@@ -51,17 +51,17 @@
         ('tab_pretty_formatted_json.json', 0),
     ),
 )
-def test_tab_pretty_format_json(filename, expected_retval):  # pragma: no cover
-    ret = pretty_format_json(['--indent', '\t', get_resource_path(filename)])
+def test_tab_main(filename, expected_retval):  # pragma: no cover
+    ret = main(['--indent', '\t', get_resource_path(filename)])
     assert ret == expected_retval
 
 
-def test_non_ascii_pretty_format_json():
-    ret = pretty_format_json(['--no-ensure-ascii', get_resource_path('non_ascii_pretty_formatted_json.json')])
+def test_non_ascii_main():
+    ret = main(['--no-ensure-ascii', get_resource_path('non_ascii_pretty_formatted_json.json')])
     assert ret == 0
 
 
-def test_autofix_pretty_format_json(tmpdir):
+def test_autofix_main(tmpdir):
     srcfile = tmpdir.join('to_be_json_formatted.json')
     shutil.copyfile(
         get_resource_path('not_pretty_formatted_json.json'),
@@ -69,30 +69,30 @@
     )
 
     # now launch the autofix on that file
-    ret = pretty_format_json(['--autofix', srcfile.strpath])
+    ret = main(['--autofix', srcfile.strpath])
     # it should have formatted it
     assert ret == 1
 
     # file was formatted (shouldn't trigger linter again)
-    ret = pretty_format_json([srcfile.strpath])
+    ret = main([srcfile.strpath])
     assert ret == 0
 
 
 def test_orderfile_get_pretty_format():
-    ret = pretty_format_json(['--top-keys=alist', get_resource_path('pretty_formatted_json.json')])
+    ret = main(['--top-keys=alist', get_resource_path('pretty_formatted_json.json')])
     assert ret == 0
 
 
 def test_not_orderfile_get_pretty_format():
-    ret = pretty_format_json(['--top-keys=blah', get_resource_path('pretty_formatted_json.json')])
+    ret = main(['--top-keys=blah', get_resource_path('pretty_formatted_json.json')])
     assert ret == 1
 
 
 def test_top_sorted_get_pretty_format():
-    ret = pretty_format_json(['--top-keys=01-alist,alist', get_resource_path('top_sorted_json.json')])
+    ret = main(['--top-keys=01-alist,alist', get_resource_path('top_sorted_json.json')])
     assert ret == 0
 
 
-def test_badfile_pretty_format_json():
-    ret = pretty_format_json([get_resource_path('ok_yaml.yaml')])
+def test_badfile_main():
+    ret = main([get_resource_path('ok_yaml.yaml')])
     assert ret == 1
diff --git a/tests/requirements_txt_fixer_test.py b/tests/requirements_txt_fixer_test.py
index 437cebd..b3a7942 100644
--- a/tests/requirements_txt_fixer_test.py
+++ b/tests/requirements_txt_fixer_test.py
@@ -1,7 +1,7 @@
 import pytest
 
 from pre_commit_hooks.requirements_txt_fixer import FAIL
-from pre_commit_hooks.requirements_txt_fixer import fix_requirements_txt
+from pre_commit_hooks.requirements_txt_fixer import main
 from pre_commit_hooks.requirements_txt_fixer import PASS
 from pre_commit_hooks.requirements_txt_fixer import Requirement
 
@@ -36,7 +36,7 @@
     path = tmpdir.join('file.txt')
     path.write_binary(input_s)
 
-    output_retval = fix_requirements_txt([path.strpath])
+    output_retval = main([path.strpath])
 
     assert path.read_binary() == output
     assert output_retval == expected_retval
@@ -44,7 +44,7 @@
 
 def test_requirement_object():
     top_of_file = Requirement()
-    top_of_file.comments.append('#foo')
+    top_of_file.comments.append(b'#foo')
     top_of_file.value = b'\n'
 
     requirement_foo = Requirement()
diff --git a/tests/sort_simple_yaml_test.py b/tests/sort_simple_yaml_test.py
index 176d12f..72f5bec 100644
--- a/tests/sort_simple_yaml_test.py
+++ b/tests/sort_simple_yaml_test.py
@@ -110,9 +110,9 @@
     lines = ['# some comment', '"a": 42', 'b: 17', '', 'c: 19']
     assert first_key(lines) == 'a": 42'
 
-    # no lines
+    # no lines (not a real situation)
     lines = []
-    assert first_key(lines) is None
+    assert first_key(lines) == ''
 
 
 @pytest.mark.parametrize('bad_lines,good_lines,_', TEST_SORTS)
diff --git a/tests/tests_should_end_in_test_test.py b/tests/tests_should_end_in_test_test.py
index dc686a5..4eb98e7 100644
--- a/tests/tests_should_end_in_test_test.py
+++ b/tests/tests_should_end_in_test_test.py
@@ -1,36 +1,36 @@
-from pre_commit_hooks.tests_should_end_in_test import validate_files
+from pre_commit_hooks.tests_should_end_in_test import main
 
 
-def test_validate_files_all_pass():
-    ret = validate_files(['foo_test.py', 'bar_test.py'])
+def test_main_all_pass():
+    ret = main(['foo_test.py', 'bar_test.py'])
     assert ret == 0
 
 
-def test_validate_files_one_fails():
-    ret = validate_files(['not_test_ending.py', 'foo_test.py'])
+def test_main_one_fails():
+    ret = main(['not_test_ending.py', 'foo_test.py'])
     assert ret == 1
 
 
-def test_validate_files_django_all_pass():
-    ret = validate_files(['--django', 'tests.py', 'test_foo.py', 'test_bar.py', 'tests/test_baz.py'])
+def test_main_django_all_pass():
+    ret = main(['--django', 'tests.py', 'test_foo.py', 'test_bar.py', 'tests/test_baz.py'])
     assert ret == 0
 
 
-def test_validate_files_django_one_fails():
-    ret = validate_files(['--django', 'not_test_ending.py', 'test_foo.py'])
+def test_main_django_one_fails():
+    ret = main(['--django', 'not_test_ending.py', 'test_foo.py'])
     assert ret == 1
 
 
 def test_validate_nested_files_django_one_fails():
-    ret = validate_files(['--django', 'tests/not_test_ending.py', 'test_foo.py'])
+    ret = main(['--django', 'tests/not_test_ending.py', 'test_foo.py'])
     assert ret == 1
 
 
-def test_validate_files_not_django_fails():
-    ret = validate_files(['foo_test.py', 'bar_test.py', 'test_baz.py'])
+def test_main_not_django_fails():
+    ret = main(['foo_test.py', 'bar_test.py', 'test_baz.py'])
     assert ret == 1
 
 
-def test_validate_files_django_fails():
-    ret = validate_files(['--django', 'foo_test.py', 'test_bar.py', 'test_baz.py'])
+def test_main_django_fails():
+    ret = main(['--django', 'foo_test.py', 'test_bar.py', 'test_baz.py'])
     assert ret == 1
diff --git a/tox.ini b/tox.ini
index c131e6f..d1e6a79 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 # These should match the travis env list
-envlist = py27,py36,py37,pypy
+envlist = py27,py36,py37,pypy3
 
 [testenv]
 deps = -rrequirements-dev.txt