diff --git a/mypy/build.py b/mypy/build.py
index 41434adf6c79..58089af97979 100644
--- a/mypy/build.py
+++ b/mypy/build.py
@@ -42,7 +42,7 @@
 from mypy.stats import dump_type_stats
 from mypy.types import Type
 from mypy.version import __version__
-from mypy.plugin import DefaultPlugin
+from mypy.plugin import Plugin, DefaultPlugin, ChainedPlugin
 
 
 # We need to know the location of this file to load data, but
@@ -183,7 +183,9 @@ def build(sources: List[BuildSource],
                            reports=reports,
                            options=options,
                            version_id=__version__,
-                           )
+                           plugin=DefaultPlugin(options.python_version))
+
+    manager.plugin = load_custom_plugins(manager.plugin, options, manager.errors)
 
     try:
         graph = dispatch(sources, manager)
@@ -333,6 +335,67 @@ def import_priority(imp: ImportBase, toplevel_priority: int) -> int:
     return toplevel_priority
 
 
+def load_custom_plugins(default_plugin: Plugin, options: Options, errors: Errors) -> Plugin:
+    """Load custom plugins if any are configured.
+
+    Return a plugin that chains all custom plugins (if any) and falls
+    back to default_plugin.
+    """
+
+    def plugin_error(message: str) -> None:
+        errors.report(0, 0, message)
+        errors.raise_error()
+
+    custom_plugins = []
+    for plugin_path in options.plugins:
+        if options.config_file:
+            # Plugin paths are relative to the config file location.
+            plugin_path = os.path.join(os.path.dirname(options.config_file), plugin_path)
+        errors.set_file(plugin_path, None)
+
+        if not os.path.isfile(plugin_path):
+            plugin_error("Can't find plugin")
+        plugin_dir = os.path.dirname(plugin_path)
+        fnam = os.path.basename(plugin_path)
+        if not fnam.endswith('.py'):
+            plugin_error("Plugin must have .py extension")
+        module_name = fnam[:-3]
+        import importlib
+        sys.path.insert(0, plugin_dir)
+        try:
+            m = importlib.import_module(module_name)
+        except Exception:
+            print('Error importing plugin {}\n'.format(plugin_path))
+            raise  # Propagate to display traceback
+        finally:
+            assert sys.path[0] == plugin_dir
+            del sys.path[0]
+        if not hasattr(m, 'plugin'):
+            plugin_error('Plugin does not define entry point function "plugin"')
+        try:
+            plugin_type = getattr(m, 'plugin')(__version__)
+        except Exception:
+            print('Error calling the plugin(version) entry point of {}\n'.format(plugin_path))
+            raise  # Propagate to display traceback
+        if not isinstance(plugin_type, type):
+            plugin_error(
+                'Type object expected as the return value of "plugin" (got {!r})'.format(
+                    plugin_type))
+        if not issubclass(plugin_type, Plugin):
+            plugin_error(
+                'Return value of "plugin" must be a subclass of "mypy.plugin.Plugin"')
+        try:
+            custom_plugins.append(plugin_type(options.python_version))
+        except Exception:
+            print('Error constructing plugin instance of {}\n'.format(plugin_type.__name__))
+            raise  # Propagate to display traceback
+    if not custom_plugins:
+        return default_plugin
+    else:
+        # Custom plugins take precendence over built-in plugins.
+        return ChainedPlugin(options.python_version, custom_plugins + [default_plugin])
+
+
 # TODO: Get rid of all_types.  It's not used except for one log message.
 #       Maybe we could instead publish a map from module ID to its type_map.
 class BuildManager:
@@ -356,6 +419,7 @@ class BuildManager:
       missing_modules: Set of modules that could not be imported encountered so far
       stale_modules:   Set of modules that needed to be rechecked
       version_id:      The current mypy version (based on commit id when possible)
+      plugin:          Active mypy plugin(s)
     """
 
     def __init__(self, data_dir: str,
@@ -364,7 +428,8 @@ def __init__(self, data_dir: str,
                  source_set: BuildSourceSet,
                  reports: Reports,
                  options: Options,
-                 version_id: str) -> None:
+                 version_id: str,
+                 plugin: Plugin) -> None:
         self.start_time = time.time()
         self.data_dir = data_dir
         self.errors = Errors(options.show_error_context, options.show_column_numbers)
@@ -384,6 +449,7 @@ def __init__(self, data_dir: str,
         self.indirection_detector = TypeIndirectionVisitor()
         self.stale_modules = set()  # type: Set[str]
         self.rechecked_modules = set()  # type: Set[str]
+        self.plugin = plugin
 
     def maybe_swap_for_shadow_path(self, path: str) -> str:
         if (self.options.shadow_file and
@@ -1506,9 +1572,8 @@ def type_check_first_pass(self) -> None:
         if self.options.semantic_analysis_only:
             return
         with self.wrap_context():
-            plugin = DefaultPlugin(self.options.python_version)
             self.type_checker = TypeChecker(manager.errors, manager.modules, self.options,
-                                            self.tree, self.xpath, plugin)
+                                            self.tree, self.xpath, manager.plugin)
             self.type_checker.check_first_pass()
 
     def type_check_second_pass(self) -> bool:
diff --git a/mypy/main.py b/mypy/main.py
index 2d0fe9ec7607..7d344530461c 100644
--- a/mypy/main.py
+++ b/mypy/main.py
@@ -372,7 +372,7 @@ def disallow_any_argument_type(raw_options: str) -> List[str]:
     parser.parse_args(args, dummy)
     config_file = dummy.config_file
     if config_file is not None and not os.path.exists(config_file):
-        parser.error("Cannot file config file '%s'" % config_file)
+        parser.error("Cannot find config file '%s'" % config_file)
 
     # Parse config file first, so command line can override.
     options = Options()
@@ -605,6 +605,7 @@ def get_init_file(dir: str) -> Optional[str]:
     # These two are for backwards compatibility
     'silent_imports': bool,
     'almost_silent': bool,
+    'plugins': lambda s: [p.strip() for p in s.split(',')],
 }
 
 SHARED_CONFIG_FILES = ('setup.cfg',)
diff --git a/mypy/options.py b/mypy/options.py
index 69f99cce9501..fac8fe6d4459 100644
--- a/mypy/options.py
+++ b/mypy/options.py
@@ -113,6 +113,9 @@ def __init__(self) -> None:
         self.debug_cache = False
         self.quick_and_dirty = False
 
+        # Paths of user plugins
+        self.plugins = []  # type: List[str]
+
         # Per-module options (raw)
         self.per_module_options = {}  # type: Dict[Pattern[str], Dict[str, object]]
 
diff --git a/mypy/plugin.py b/mypy/plugin.py
index 5015f7b4c940..7acd4d0b29a5 100644
--- a/mypy/plugin.py
+++ b/mypy/plugin.py
@@ -1,4 +1,4 @@
-from typing import Callable, List, Tuple, Optional, NamedTuple
+from typing import Callable, List, Tuple, Optional, NamedTuple, TypeVar
 
 from mypy.nodes import Expression, StrExpr, IntExpr, UnaryExpr, Context
 from mypy.types import (
@@ -60,7 +60,7 @@
 
 
 class Plugin:
-    """Base class of type checker plugins.
+    """Base class of all type checker plugins.
 
     This defines a no-op plugin.  Subclasses can override some methods to
     provide some actual functionality.
@@ -69,8 +69,6 @@ class Plugin:
     results might be cached).
     """
 
-    # TODO: Way of chaining multiple plugins
-
     def __init__(self, python_version: Tuple[int, int]) -> None:
         self.python_version = python_version
 
@@ -86,6 +84,46 @@ def get_method_hook(self, fullname: str) -> Optional[MethodHook]:
     # TODO: metaclass / class decorator hook
 
 
+T = TypeVar('T')
+
+
+class ChainedPlugin(Plugin):
+    """A plugin that represents a sequence of chained plugins.
+
+    Each lookup method returns the hook for the first plugin that
+    reports a match.
+
+    This class should not be subclassed -- use Plugin as the base class
+    for all plugins.
+    """
+
+    # TODO: Support caching of lookup results (through a LRU cache, for example).
+
+    def __init__(self, python_version: Tuple[int, int], plugins: List[Plugin]) -> None:
+        """Initialize chained plugin.
+
+        Assume that the child plugins aren't mutated (results may be cached).
+        """
+        super().__init__(python_version)
+        self._plugins = plugins
+
+    def get_function_hook(self, fullname: str) -> Optional[FunctionHook]:
+        return self._find_hook(lambda plugin: plugin.get_function_hook(fullname))
+
+    def get_method_signature_hook(self, fullname: str) -> Optional[MethodSignatureHook]:
+        return self._find_hook(lambda plugin: plugin.get_method_signature_hook(fullname))
+
+    def get_method_hook(self, fullname: str) -> Optional[MethodHook]:
+        return self._find_hook(lambda plugin: plugin.get_method_hook(fullname))
+
+    def _find_hook(self, lookup: Callable[[Plugin], T]) -> Optional[T]:
+        for plugin in self._plugins:
+            hook = lookup(plugin)
+            if hook:
+                return hook
+        return None
+
+
 class DefaultPlugin(Plugin):
     """Type checker plugin that is enabled by default."""
 
diff --git a/mypy/test/data.py b/mypy/test/data.py
index ccee92eac276..09fe931d0c62 100644
--- a/mypy/test/data.py
+++ b/mypy/test/data.py
@@ -13,6 +13,9 @@
 from mypy.myunit import TestCase, SkipTestCaseException
 
 
+root_dir = os.path.normpath(os.path.join(os.path.dirname(__file__), '..', '..'))
+
+
 def parse_test_cases(
         path: str,
         perform: Optional[Callable[['DataDrivenTestCase'], None]],
@@ -62,7 +65,9 @@ def parse_test_cases(
                     # Record an extra file needed for the test case.
                     arg = p[i].arg
                     assert arg is not None
-                    file_entry = (join(base_path, arg), '\n'.join(p[i].data))
+                    contents = '\n'.join(p[i].data)
+                    contents = expand_variables(contents)
+                    file_entry = (join(base_path, arg), contents)
                     if p[i].id == 'file':
                         files.append(file_entry)
                     elif p[i].id == 'outfile':
@@ -119,13 +124,15 @@ def parse_test_cases(
                     deleted_paths.setdefault(num, set()).add(full)
                 elif p[i].id == 'out' or p[i].id == 'out1':
                     tcout = p[i].data
-                    if native_sep and os.path.sep == '\\':
+                    tcout = [expand_variables(line) for line in tcout]
+                    if os.path.sep == '\\':
                         tcout = [fix_win_path(line) for line in tcout]
                     ok = True
                 elif re.match(r'out[0-9]*$', p[i].id):
                     passnum = int(p[i].id[3:])
                     assert passnum > 1
                     output = p[i].data
+                    output = [expand_variables(line) for line in output]
                     if native_sep and os.path.sep == '\\':
                         output = [fix_win_path(line) for line in output]
                     tcout2[passnum] = output
@@ -415,6 +422,10 @@ def expand_includes(a: List[str], base_path: str) -> List[str]:
     return res
 
 
+def expand_variables(s: str) -> str:
+    return s.replace('<ROOT>', root_dir)
+
+
 def expand_errors(input: List[str], output: List[str], fnam: str) -> None:
     """Transform comments such as '# E: message' or
     '# E:3: message' in input.
@@ -445,16 +456,17 @@ def expand_errors(input: List[str], output: List[str], fnam: str) -> None:
 
 
 def fix_win_path(line: str) -> str:
-    r"""Changes paths to Windows paths in error messages.
+    r"""Changes Windows paths to Linux paths in error messages.
 
-    E.g. foo/bar.py -> foo\bar.py.
+    E.g. foo\bar.py -> foo/bar.py.
     """
+    line = line.replace(root_dir, root_dir.replace('\\', '/'))
     m = re.match(r'^([\S/]+):(\d+:)?(\s+.*)', line)
     if not m:
         return line
     else:
         filename, lineno, message = m.groups()
-        return '{}:{}{}'.format(filename.replace('/', '\\'),
+        return '{}:{}{}'.format(filename.replace('\\', '/'),
                                 lineno or '', message)
 
 
diff --git a/mypy/test/testcheck.py b/mypy/test/testcheck.py
index 91a818ac0f01..b01424b8a053 100644
--- a/mypy/test/testcheck.py
+++ b/mypy/test/testcheck.py
@@ -76,6 +76,7 @@
     'check-classvar.test',
     'check-enum.test',
     'check-incomplete-fixture.test',
+    'check-custom-plugin.test',
 ]
 
 
@@ -261,7 +262,8 @@ def find_error_paths(self, a: List[str]) -> Set[str]:
         for line in a:
             m = re.match(r'([^\s:]+):\d+: error:', line)
             if m:
-                p = m.group(1).replace('/', os.path.sep)
+                # Normalize to Linux paths.
+                p = m.group(1).replace(os.path.sep, '/')
                 hits.add(p)
         return hits
 
diff --git a/mypy/test/testcmdline.py b/mypy/test/testcmdline.py
index 8e56b42bc766..06009f107b14 100644
--- a/mypy/test/testcmdline.py
+++ b/mypy/test/testcmdline.py
@@ -15,7 +15,7 @@
 from mypy.test.config import test_data_prefix, test_temp_dir
 from mypy.test.data import fix_cobertura_filename
 from mypy.test.data import parse_test_cases, DataDrivenTestCase
-from mypy.test.helpers import assert_string_arrays_equal
+from mypy.test.helpers import assert_string_arrays_equal, normalize_error_messages
 from mypy.version import __version__, base_version
 
 # Path to Python 3 interpreter
@@ -71,10 +71,12 @@ def test_python_evaluation(testcase: DataDrivenTestCase) -> None:
                                                       os.path.abspath(test_temp_dir))
             if testcase.native_sep and os.path.sep == '\\':
                 normalized_output = [fix_cobertura_filename(line) for line in normalized_output]
+            normalized_output = normalize_error_messages(normalized_output)
             assert_string_arrays_equal(expected_content.splitlines(), normalized_output,
                                        'Output file {} did not match its expected output'.format(
                                            path))
     else:
+        out = normalize_error_messages(out)
         assert_string_arrays_equal(testcase.output, out,
                                    'Invalid output ({}, line {})'.format(
                                        testcase.file, testcase.line))
diff --git a/mypy/test/testgraph.py b/mypy/test/testgraph.py
index 7a9062914f89..d168ad53e236 100644
--- a/mypy/test/testgraph.py
+++ b/mypy/test/testgraph.py
@@ -8,6 +8,8 @@
 from mypy.version import __version__
 from mypy.options import Options
 from mypy.report import Reports
+from mypy.plugin import Plugin
+from mypy import defaults
 
 
 class GraphSuite(Suite):
@@ -42,6 +44,7 @@ def _make_manager(self) -> BuildManager:
             reports=Reports('', {}),
             options=Options(),
             version_id=__version__,
+            plugin=Plugin(defaults.PYTHON3_VERSION),
         )
         return manager
 
diff --git a/mypy/test/testsemanal.py b/mypy/test/testsemanal.py
index 99c0078e9196..6d7f2ddb24bb 100644
--- a/mypy/test/testsemanal.py
+++ b/mypy/test/testsemanal.py
@@ -89,6 +89,7 @@ def test_semanal(testcase: DataDrivenTestCase) -> None:
                 a += str(f).split('\n')
     except CompileError as e:
         a = e.messages
+    a = normalize_error_messages(a)
     assert_string_arrays_equal(
         testcase.output, a,
         'Invalid semantic analyzer output ({}, line {})'.format(testcase.file,
diff --git a/mypy/test/testtransform.py b/mypy/test/testtransform.py
index 1dac3081efbd..0dcdd1d0c649 100644
--- a/mypy/test/testtransform.py
+++ b/mypy/test/testtransform.py
@@ -7,7 +7,9 @@
 from mypy import build
 from mypy.build import BuildSource
 from mypy.myunit import Suite
-from mypy.test.helpers import assert_string_arrays_equal, testfile_pyversion
+from mypy.test.helpers import (
+    assert_string_arrays_equal, testfile_pyversion, normalize_error_messages
+)
 from mypy.test.data import parse_test_cases, DataDrivenTestCase
 from mypy.test.config import test_data_prefix, test_temp_dir
 from mypy.errors import CompileError
@@ -73,6 +75,7 @@ def test_transform(testcase: DataDrivenTestCase) -> None:
                 a += str(f).split('\n')
     except CompileError as e:
         a = e.messages
+    a = normalize_error_messages(a)
     assert_string_arrays_equal(
         testcase.output, a,
         'Invalid semantic analyzer output ({}, line {})'.format(testcase.file,
diff --git a/test-data/unit/check-custom-plugin.test b/test-data/unit/check-custom-plugin.test
new file mode 100644
index 000000000000..30b00a4b3a62
--- /dev/null
+++ b/test-data/unit/check-custom-plugin.test
@@ -0,0 +1,71 @@
+-- Test cases for user-defined plugins
+--
+-- Note: Plugins used by tests live under test-data/unit/plugins. Defining
+--       plugin files in test cases does not work reliably.
+
+[case testFunctionPlugin]
+# flags: --config-file tmp/mypy.ini
+def f() -> str: ...
+reveal_type(f())  # E: Revealed type is 'builtins.int'
+[file mypy.ini]
+[[mypy]
+plugins=<ROOT>/test-data/unit/plugins/fnplugin.py
+
+[case testTwoPlugins]
+# flags: --config-file tmp/mypy.ini
+def f(): ...
+def g(): ...
+def h(): ...
+reveal_type(f())  # E: Revealed type is 'builtins.int'
+reveal_type(g())  # E: Revealed type is 'builtins.str'
+reveal_type(h())  # E: Revealed type is 'Any'
+[file mypy.ini]
+[[mypy]
+plugins=<ROOT>/test-data/unit/plugins/fnplugin.py,
+  <ROOT>/test-data/unit/plugins/plugin2.py
+
+[case testMissingPlugin]
+# flags: --config-file tmp/mypy.ini
+[file mypy.ini]
+[[mypy]
+plugins=missing.py
+[out]
+tmp/missing.py:0: error: Can't find plugin
+--' (work around syntax highlighting)
+
+[case testInvalidPluginExtension]
+# flags: --config-file tmp/mypy.ini
+[file mypy.ini]
+[[mypy]
+plugins=badext.pyi
+[file badext.pyi]
+[out]
+tmp/badext.pyi:0: error: Plugin must have .py extension
+
+[case testMissingPluginEntryPoint]
+# flags: --config-file tmp/mypy.ini
+[file mypy.ini]
+[[mypy]
+plugins=<ROOT>/test-data/unit/plugins/noentry.py
+[out]
+<ROOT>/test-data/unit/plugins/noentry.py:0: error: Plugin does not define entry point function "plugin"
+
+[case testInvalidPluginEntryPointReturnValue]
+# flags: --config-file tmp/mypy.ini
+def f(): pass
+f()
+[file mypy.ini]
+[[mypy]
+plugins=<ROOT>/test-data/unit/plugins/badreturn.py
+[out]
+<ROOT>/test-data/unit/plugins/badreturn.py:0: error: Type object expected as the return value of "plugin" (got None)
+
+[case testInvalidPluginEntryPointReturnValue2]
+# flags: --config-file tmp/mypy.ini
+def f(): pass
+f()
+[file mypy.ini]
+[[mypy]
+plugins=<ROOT>/test-data/unit/plugins/badreturn2.py
+[out]
+<ROOT>/test-data/unit/plugins/badreturn2.py:0: error: Return value of "plugin" must be a subclass of "mypy.plugin.Plugin"
diff --git a/test-data/unit/plugins/badreturn.py b/test-data/unit/plugins/badreturn.py
new file mode 100644
index 000000000000..fd7430606dd6
--- /dev/null
+++ b/test-data/unit/plugins/badreturn.py
@@ -0,0 +1,2 @@
+def plugin(version):
+    pass
diff --git a/test-data/unit/plugins/badreturn2.py b/test-data/unit/plugins/badreturn2.py
new file mode 100644
index 000000000000..c7e0447841c1
--- /dev/null
+++ b/test-data/unit/plugins/badreturn2.py
@@ -0,0 +1,5 @@
+class MyPlugin:
+    pass
+
+def plugin(version):
+    return MyPlugin
diff --git a/test-data/unit/plugins/fnplugin.py b/test-data/unit/plugins/fnplugin.py
new file mode 100644
index 000000000000..d5027219a09f
--- /dev/null
+++ b/test-data/unit/plugins/fnplugin.py
@@ -0,0 +1,13 @@
+from mypy.plugin import Plugin
+
+class MyPlugin(Plugin):
+    def get_function_hook(self, fullname):
+        if fullname == '__main__.f':
+            return my_hook
+        return None
+
+def my_hook(arg_types, args, inferred_return_type, named_generic_type):
+    return named_generic_type('builtins.int', [])
+
+def plugin(version):
+    return MyPlugin
diff --git a/test-data/unit/plugins/noentry.py b/test-data/unit/plugins/noentry.py
new file mode 100644
index 000000000000..c591ad11fd64
--- /dev/null
+++ b/test-data/unit/plugins/noentry.py
@@ -0,0 +1 @@
+# empty plugin
diff --git a/test-data/unit/plugins/plugin2.py b/test-data/unit/plugins/plugin2.py
new file mode 100644
index 000000000000..1584871fae1d
--- /dev/null
+++ b/test-data/unit/plugins/plugin2.py
@@ -0,0 +1,13 @@
+from mypy.plugin import Plugin
+
+class Plugin2(Plugin):
+    def get_function_hook(self, fullname):
+        if fullname in ('__main__.f', '__main__.g'):
+            return str_hook
+        return None
+
+def str_hook(arg_types, args, inferred_return_type, named_generic_type):
+    return named_generic_type('builtins.str', [])
+
+def plugin(version):
+    return Plugin2