diff --git a/pre_commit/clientlib/validate_base.py b/pre_commit/clientlib/validate_base.py
index 75a4cf96..dcf7f115 100644
--- a/pre_commit/clientlib/validate_base.py
+++ b/pre_commit/clientlib/validate_base.py
@@ -4,6 +4,8 @@ import jsonschema.exceptions
 import os.path
 import yaml
 
+from pre_commit.jsonschema_extensions import apply_defaults
+
 
 def get_validator(
     json_schema,
@@ -39,6 +41,8 @@ def get_validator(
                 'File {0} is not a valid file'.format(filename), e,
             )
 
+        obj = apply_defaults(obj, json_schema)
+
         additional_validation_strategy(obj)
 
         return obj
diff --git a/pre_commit/clientlib/validate_config.py b/pre_commit/clientlib/validate_config.py
index 0f783a6e..307b5633 100644
--- a/pre_commit/clientlib/validate_config.py
+++ b/pre_commit/clientlib/validate_config.py
@@ -28,9 +28,10 @@ CONFIG_JSON_SCHEMA = {
                     'properties': {
                         'id': {'type': 'string'},
                         'files': {'type': 'string'},
+                        'exclude': {'type': 'string', 'default': '^$'},
                         'args': {
                             'type': 'array',
-                            'minItems': 1,
+                            'default': [],
                             'items': {'type': 'string'},
                         },
                     },
@@ -43,17 +44,22 @@ CONFIG_JSON_SCHEMA = {
 }
 
 
+def try_regex(repo, hook, value, field_name):
+    try:
+        re.compile(value)
+    except re.error:
+        raise InvalidConfigError(
+            'Invalid {0} regex at {1}, {2}: {3}'.format(
+                field_name, repo, hook, value,
+            )
+        )
+
+
 def validate_config_extra(config):
     for repo in config:
         for hook in repo['hooks']:
-            try:
-                re.compile(hook['files'])
-            except re.error:
-                raise InvalidConfigError(
-                    'Invalid file regex at {0}, {1}: {2}'.format(
-                        repo['repo'], hook['id'], hook['files'],
-                    )
-                )
+            try_regex(repo, hook['id'], hook['files'], 'files')
+            try_regex(repo, hook['id'], hook['exclude'], 'exclude')
 
 
 load_config = get_validator(
diff --git a/pre_commit/clientlib/validate_manifest.py b/pre_commit/clientlib/validate_manifest.py
index 43200832..3f89cfae 100644
--- a/pre_commit/clientlib/validate_manifest.py
+++ b/pre_commit/clientlib/validate_manifest.py
@@ -20,10 +20,10 @@ MANIFEST_JSON_SCHEMA = {
         'properties': {
             'id': {'type': 'string'},
             'name': {'type': 'string'},
-            'description': {'type': 'string'},
+            'description': {'type': 'string', 'default': ''},
             'entry': {'type': 'string'},
             'language': {'type': 'string'},
-            'expected_return_value': {'type': 'number'},
+            'expected_return_value': {'type': 'number', 'default': 0},
         },
         'required': ['id', 'name', 'entry', 'language'],
     },
@@ -32,11 +32,9 @@ MANIFEST_JSON_SCHEMA = {
 
 def additional_manifest_check(obj):
     for hook_config in obj:
-        language = hook_config.get('language')
+        language = hook_config['language']
 
-        if language is not None and not any(
-            language.startswith(lang) for lang in all_languages
-        ):
+        if not any(language.startswith(lang) for lang in all_languages):
             raise InvalidManifestError(
                 'Expected language {0} for {1} to start with one of {2!r}'.format(
                     hook_config['id'],
diff --git a/pre_commit/git.py b/pre_commit/git.py
index 7a50ead2..692dbc23 100644
--- a/pre_commit/git.py
+++ b/pre_commit/git.py
@@ -40,12 +40,16 @@ def get_all_files():
 def get_files_matching(all_file_list_strategy):
     @functools.wraps(all_file_list_strategy)
     @memoize_by_cwd
-    def wrapper(expr):
-        regex = re.compile(expr)
+    def wrapper(include_expr, exclude_expr):
+        include_regex = re.compile(include_expr)
+        exclude_regex = re.compile(exclude_expr)
         return set(filter(os.path.exists, (
             filename
             for filename in all_file_list_strategy()
-            if regex.search(filename)
+            if (
+                include_regex.search(filename) and
+                not exclude_regex.search(filename)
+            )
         )))
     return wrapper
 
diff --git a/pre_commit/jsonschema_extensions.py b/pre_commit/jsonschema_extensions.py
new file mode 100644
index 00000000..938303c9
--- /dev/null
+++ b/pre_commit/jsonschema_extensions.py
@@ -0,0 +1,32 @@
+
+import copy
+import jsonschema
+import jsonschema.validators
+
+
+# From https://github.com/Julian/jsonschema/blob/master/docs/faq.rst
+def extend_with_default(validator_class):
+    validate_properties = validator_class.VALIDATORS["properties"]
+
+    def set_defaults(validator, properties, instance, schema):
+        for error in validate_properties(
+            validator, properties, instance, schema,
+        ):
+            yield error
+
+        for property, subschema in properties.iteritems():
+            if "default" in subschema:
+                instance.setdefault(property, subschema["default"])
+
+    return jsonschema.validators.extend(
+        validator_class, {"properties" : set_defaults},
+    )
+
+
+DefaultingValidator = extend_with_default(jsonschema.Draft4Validator)
+
+
+def apply_defaults(obj, schema):
+    obj = copy.deepcopy(obj)
+    DefaultingValidator(schema).validate(obj)
+    return obj
diff --git a/pre_commit/languages/helpers.py b/pre_commit/languages/helpers.py
index 4c05f3e7..fab8e5ae 100644
--- a/pre_commit/languages/helpers.py
+++ b/pre_commit/languages/helpers.py
@@ -2,7 +2,7 @@
 
 def run_hook(env, hook, file_args):
     return env.run(
-        ' '.join(['xargs', hook['entry']] + hook.get('args', [])),
+        ' '.join(['xargs', hook['entry']] + hook['args']),
         stdin='\n'.join(list(file_args) + ['']),
         retcode=None,
     )
diff --git a/pre_commit/languages/script.py b/pre_commit/languages/script.py
index 783743ae..e622a7c8 100644
--- a/pre_commit/languages/script.py
+++ b/pre_commit/languages/script.py
@@ -6,7 +6,7 @@ def install_environment(repo_cmd_runner):
 
 def run_hook(repo_cmd_runner, hook, file_args):
     return repo_cmd_runner.run(
-        ['xargs', '{{prefix}}{0}'.format(hook['entry'])] + hook.get('args', []),
+        ['xargs', '{{prefix}}{0}'.format(hook['entry'])] + hook['args'],
         # TODO: this is duplicated in pre_commit/languages/helpers.py
         stdin='\n'.join(list(file_args) + ['']),
         retcode=None,
diff --git a/pre_commit/languages/system.py b/pre_commit/languages/system.py
index 8439c091..39dcf89c 100644
--- a/pre_commit/languages/system.py
+++ b/pre_commit/languages/system.py
@@ -6,7 +6,7 @@ def install_environment(repo_cmd_runner):
 
 def run_hook(repo_cmd_runner, hook, file_args):
     return repo_cmd_runner.run(
-        ['xargs', hook['entry']] + hook.get('args', []),
+        ['xargs', hook['entry']] + hook['args'],
         # TODO: this is duplicated in pre_commit/languages/helpers.py
         stdin='\n'.join(list(file_args) + ['']),
         retcode=None,
diff --git a/pre_commit/repository.py b/pre_commit/repository.py
index f8f4d1c2..10ff2b6c 100644
--- a/pre_commit/repository.py
+++ b/pre_commit/repository.py
@@ -27,9 +27,7 @@ class Repository(object):
 
     @cached_property
     def languages(self):
-        return set(filter(None, (
-            hook.get('language') for hook in self.hooks.values()
-        )))
+        return set(hook['language'] for hook in self.hooks.values())
 
     @cached_property
     def hooks(self):
diff --git a/pre_commit/run.py b/pre_commit/run.py
index 63247658..14974313 100644
--- a/pre_commit/run.py
+++ b/pre_commit/run.py
@@ -35,10 +35,10 @@ def _run_single_hook(runner, repository, hook_id, all_files=False):
     retcode, stdout, stderr = repository.run_hook(
         runner.cmd_runner,
         hook_id,
-        get_filenames(hook['files']),
+        get_filenames(hook['files'], hook['exclude']),
     )
 
-    if retcode != repository.hooks[hook_id].get('expected_return_value', 0):
+    if retcode != repository.hooks[hook_id]['expected_return_value']:
         output = '\n'.join([stdout, stderr]).strip()
         retcode = 1
         color = RED
diff --git a/tests/clientlib/validate_config_test.py b/tests/clientlib/validate_config_test.py
index bf80793f..f3ccc4df 100644
--- a/tests/clientlib/validate_config_test.py
+++ b/tests/clientlib/validate_config_test.py
@@ -7,6 +7,7 @@ from pre_commit.clientlib.validate_config import CONFIG_JSON_SCHEMA
 from pre_commit.clientlib.validate_config import InvalidConfigError
 from pre_commit.clientlib.validate_config import run
 from pre_commit.clientlib.validate_config import validate_config_extra
+from pre_commit.jsonschema_extensions import apply_defaults
 
 
 def test_returns_0_for_valid_config():
@@ -35,29 +36,40 @@ def is_valid_according_to_schema(obj, schema):
         [{
           'repo': 'git@github.com:pre-commit/pre-commit-hooks',
           'sha': 'cd74dc150c142c3be70b24eaf0b02cae9d235f37',
-          'hooks': [
-              {
-                  'id': 'pyflakes',
-                  'files': '*.py',
-              }
-          ]
+          'hooks': [{'id': 'pyflakes', 'files': '\.py$'}],
         }],
         True,
     ),
     (
         [{
-          'repo': 'git@github.com:pre-commit/pre-commit-hooks',
-          'sha': 'cd74dc150c142c3be70b24eaf0b02cae9d235f37',
-          'hooks': [
-              {
-                  'id': 'pyflakes',
-                  'files': '*.py',
-                  'args': ['foo', 'bar', 'baz'],
-              }
-          ]
+            'repo': 'git@github.com:pre-commit/pre-commit-hooks',
+            'sha': 'cd74dc150c142c3be70b24eaf0b02cae9d235f37',
+            'hooks': [
+                {
+                    'id': 'pyflakes',
+                    'files': '\.py$',
+                    'args': ['foo', 'bar', 'baz'],
+                },
+            ],
         }],
         True,
     ),
+    (
+        [{
+            'repo': 'git@github.com:pre-commit/pre-commit-hooks',
+            'sha': 'cd74dc150c142c3be70b24eaf0b02cae9d235f37',
+            'hooks': [
+                {
+                    'id': 'pyflakes',
+                    'files': '\.py$',
+                    # Exclude pattern must be a string
+                    'exclude': 0,
+                    'args': ['foo', 'bar', 'baz'],
+                },
+            ],
+        }],
+        False,
+    ),
 ))
 def test_is_valid_according_to_schema(manifest_obj, expected):
     ret = is_valid_according_to_schema(manifest_obj, CONFIG_JSON_SCHEMA)
@@ -67,12 +79,50 @@ def test_is_valid_according_to_schema(manifest_obj, expected):
 def test_config_with_failing_regexes_fails():
     with pytest.raises(InvalidConfigError):
         # Note the regex '(' is invalid (unbalanced parens)
-        validate_config_extra(
-            [{'repo': 'foo', 'hooks': [{'id': 'hook_id', 'files': '('}]}]
+        config = apply_defaults(
+            [{
+                'repo': 'foo',
+                'sha': 'foo',
+                'hooks': [{'id': 'hook_id', 'files': '('}],
+            }],
+            CONFIG_JSON_SCHEMA,
         )
+        validate_config_extra(config)
 
 
 def test_config_with_ok_regexes_passes():
-    validate_config_extra(
-        [{'repo': 'foo', 'hooks': [{'id': 'hook_id', 'files': '\.py$'}]}]
+    config = apply_defaults(
+        [{
+            'repo': 'foo',
+            'sha': 'foo',
+            'hooks': [{'id': 'hook_id', 'files': '\.py$'}],
+        }],
+        CONFIG_JSON_SCHEMA,
     )
+    validate_config_extra(config)
+
+
+def test_config_with_invalid_exclude_regex_fails():
+    with pytest.raises(InvalidConfigError):
+        # Note the regex '(' is invalid (unbalanced parens)
+        config = apply_defaults(
+            [{
+                'repo': 'foo',
+                'sha': 'foo',
+                'hooks': [{'id': 'hook_id', 'files': '', 'exclude': '('}],
+            }],
+            CONFIG_JSON_SCHEMA,
+        )
+        validate_config_extra(config)
+
+
+def test_config_with_ok_exclude_regex_passes():
+    config = apply_defaults(
+        [{
+            'repo': 'foo',
+            'sha': 'foo',
+            'hooks': [{'id': 'hook_id', 'files': '', 'exclude': '^vendor/'}],
+        }],
+        CONFIG_JSON_SCHEMA,
+    )
+    validate_config_extra(config)
diff --git a/tests/clientlib/validate_manifest_test.py b/tests/clientlib/validate_manifest_test.py
index 7d64db28..a1c10580 100644
--- a/tests/clientlib/validate_manifest_test.py
+++ b/tests/clientlib/validate_manifest_test.py
@@ -27,11 +27,10 @@ def test_additional_manifest_check_raises_for_bad_language():
 
 
 @pytest.mark.parametrize(('obj'), (
-    [{}],
     [{'language': 'python'}],
     [{'language': 'ruby'}],
 ))
-def test_additional_manifest_check_is_ok_with_missing_language(obj):
+def test_additional_manifest_check_languages(obj):
     additional_manifest_check(obj)
 
 
diff --git a/tests/commands_test.py b/tests/commands_test.py
index 6dba08d0..21740f2c 100644
--- a/tests/commands_test.py
+++ b/tests/commands_test.py
@@ -1,5 +1,4 @@
 
-import jsonschema
 import os
 import os.path
 import pkg_resources
@@ -18,6 +17,7 @@ from pre_commit.commands import install
 from pre_commit.commands import RepositoryCannotBeUpdatedError
 from pre_commit.commands import uninstall
 from pre_commit.commands import _update_repository
+from pre_commit.jsonschema_extensions import apply_defaults
 from pre_commit.ordereddict import OrderedDict
 from pre_commit.runner import Runner
 from pre_commit.yaml_extensions import ordered_dump
@@ -60,8 +60,9 @@ def up_to_date_repo(python_hooks_repo):
         ('sha', git.get_head_sha(python_hooks_repo)),
         ('hooks', [OrderedDict((('id', 'foo'), ('files', '')))]),
     ))
-    jsonschema.validate([config], CONFIG_JSON_SCHEMA)
-    validate_config_extra([config])
+    wrapped_config = apply_defaults([config], CONFIG_JSON_SCHEMA)
+    validate_config_extra(wrapped_config)
+    config = wrapped_config[0]
 
     with open(os.path.join(python_hooks_repo, C.CONFIG_FILE), 'w') as file_obj:
         file_obj.write(
@@ -96,8 +97,9 @@ def out_of_date_repo(python_hooks_repo):
         ('sha', git.get_head_sha(python_hooks_repo)),
         ('hooks', [OrderedDict((('id', 'foo'), ('files', '')))]),
     ))
-    jsonschema.validate([config], CONFIG_JSON_SCHEMA)
-    validate_config_extra([config])
+    config_wrapped = apply_defaults([config], CONFIG_JSON_SCHEMA)
+    validate_config_extra(config_wrapped)
+    config = config_wrapped[0]
     local['git']['commit', '--allow-empty', '-m', 'foo']()
     head_sha = git.get_head_sha(python_hooks_repo)
 
@@ -135,8 +137,9 @@ def hook_disappearing_repo(python_hooks_repo):
         ('sha', git.get_head_sha(python_hooks_repo)),
         ('hooks', [OrderedDict((('id', 'foo'), ('files', '')))]),
     ))
-    jsonschema.validate([config], CONFIG_JSON_SCHEMA)
-    validate_config_extra([config])
+    config_wrapped = apply_defaults([config], CONFIG_JSON_SCHEMA)
+    validate_config_extra(config_wrapped)
+    config = config_wrapped[0]
     shutil.copy(get_resource_path('manifest_without_foo.yaml'), C.MANIFEST_FILE)
     local['git']['add', '.']()
     local['git']['commit', '-m', 'Remove foo']()
diff --git a/tests/conftest.py b/tests/conftest.py
index 70e3a9a1..feecc6da 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,6 +1,5 @@
 from __future__ import absolute_import
 
-import jsonschema
 import pytest
 import time
 from plumbum import local
@@ -8,6 +7,7 @@ from plumbum import local
 from pre_commit import git
 from pre_commit.clientlib.validate_config import CONFIG_JSON_SCHEMA
 from pre_commit.clientlib.validate_config import validate_config_extra
+from pre_commit.jsonschema_extensions import apply_defaults
 from testing.util import copy_tree_to_path
 from testing.util import get_resource_path
 
@@ -69,9 +69,9 @@ def _make_config(path, hook_id, file_regex):
         'sha': git.get_head_sha(path),
         'hooks': [{'id': hook_id, 'files': file_regex}],
     }
-    jsonschema.validate([config], CONFIG_JSON_SCHEMA)
-    validate_config_extra([config])
-    return config
+    config_wrapped = apply_defaults([config], CONFIG_JSON_SCHEMA)
+    validate_config_extra(config_wrapped)
+    return config_wrapped[0]
 
 
 @pytest.yield_fixture
diff --git a/tests/git_test.py b/tests/git_test.py
index 17d60311..a6ecc574 100644
--- a/tests/git_test.py
+++ b/tests/git_test.py
@@ -29,7 +29,7 @@ def get_files_matching_func():
 
 
 def test_get_files_matching_base(get_files_matching_func):
-    ret = get_files_matching_func('')
+    ret = get_files_matching_func('', '^$')
     assert ret == set([
         'pre_commit/run.py',
         'pre_commit/git.py',
@@ -38,7 +38,7 @@ def test_get_files_matching_base(get_files_matching_func):
 
 
 def test_get_files_matching_total_match(get_files_matching_func):
-    ret = get_files_matching_func('^.*\.py$')
+    ret = get_files_matching_func('^.*\.py$', '^$')
     assert ret == set([
         'pre_commit/run.py',
         'pre_commit/git.py',
@@ -46,10 +46,15 @@ def test_get_files_matching_total_match(get_files_matching_func):
 
 
 def test_does_search_instead_of_match(get_files_matching_func):
-    ret = get_files_matching_func('\.yaml$')
+    ret = get_files_matching_func('\.yaml$', '^$')
     assert ret == set(['hooks.yaml'])
 
 
 def test_does_not_include_deleted_fileS(get_files_matching_func):
-    ret = get_files_matching_func('exist.py')
+    ret = get_files_matching_func('exist.py', '^$')
     assert ret == set()
+
+
+def test_exclude_removes_files(get_files_matching_func):
+    ret = get_files_matching_func('', '\.py$')
+    assert ret == set(['hooks.yaml'])
diff --git a/tests/jsonschema_extensions_test.py b/tests/jsonschema_extensions_test.py
new file mode 100644
index 00000000..4ea2686c
--- /dev/null
+++ b/tests/jsonschema_extensions_test.py
@@ -0,0 +1,51 @@
+
+from pre_commit.jsonschema_extensions import apply_defaults
+
+
+def test_apply_defaults_copies_object():
+    input = {}
+    ret = apply_defaults(input, {})
+    assert ret is not input
+
+
+def test_apply_default_does_not_touch_schema_without_defaults():
+    ret = apply_defaults(
+        {'foo': 'bar'},
+        {'type': 'object', 'properties': {'foo': {}, 'baz': {}}},
+    )
+    assert ret == {'foo': 'bar'}
+
+
+def test_apply_defaults_applies_defaults():
+    ret = apply_defaults(
+        {'foo': 'bar'},
+        {
+            'type': 'object',
+            'properties': {
+                'foo': {'default': 'biz'},
+                'baz': {'default': 'herp'},
+            }
+        }
+    )
+    assert ret == {'foo': 'bar', 'baz': 'herp'}
+
+
+def test_apply_defaults_deep():
+    ret = apply_defaults(
+        {'foo': {'bar': {}}},
+        {
+            'type': 'object',
+            'properties': {
+                'foo': {
+                    'type': 'object',
+                    'properties': {
+                        'bar': {
+                            'type': 'object',
+                            'properties': {'baz': {'default': 'herp'}},
+                        },
+                    },
+                },
+            },
+        },
+    )
+    assert ret == {'foo': {'bar': {'baz': 'herp'}}}
diff --git a/tests/repository_test.py b/tests/repository_test.py
index 0b4011e7..2276b224 100644
--- a/tests/repository_test.py
+++ b/tests/repository_test.py
@@ -1,11 +1,11 @@
 import os
-import jsonschema
 import pytest
 
 import pre_commit.constants as C
 from pre_commit import git
 from pre_commit.clientlib.validate_config import CONFIG_JSON_SCHEMA
 from pre_commit.clientlib.validate_config import validate_config_extra
+from pre_commit.jsonschema_extensions import apply_defaults
 from pre_commit.prefixed_command_runner import PrefixedCommandRunner
 from pre_commit.repository import Repository
 
@@ -111,9 +111,9 @@ def mock_repo_config():
             'files': '\.py$',
         }],
     }
-    jsonschema.validate([config], CONFIG_JSON_SCHEMA)
-    validate_config_extra([config])
-    return config
+    config_wrapped = apply_defaults([config], CONFIG_JSON_SCHEMA)
+    validate_config_extra(config_wrapped)
+    return config_wrapped[0]
 
 
 def test_repo_url(mock_repo_config):