diff --git a/.gitignore b/.gitignore
index e53d4908d..747b8714d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,6 +10,7 @@
 *.tac
 _trial_temp/
 _trial_temp*/
+/out
 
 # stuff that is likely to exist when you run a server locally
 /*.db
diff --git a/changelog.d/6150.misc b/changelog.d/6150.misc
new file mode 100644
index 000000000..a373c091a
--- /dev/null
+++ b/changelog.d/6150.misc
@@ -0,0 +1 @@
+Expand type-checking on modules imported by synapse.config.
diff --git a/synapse/api/errors.py b/synapse/api/errors.py
index cf1ebf1af..1bb2e8678 100644
--- a/synapse/api/errors.py
+++ b/synapse/api/errors.py
@@ -17,6 +17,7 @@
 """Contains exceptions and error codes."""
 
 import logging
+from typing import Dict
 
 from six import iteritems
 from six.moves import http_client
@@ -111,7 +112,7 @@ class ProxiedRequestError(SynapseError):
     def __init__(self, code, msg, errcode=Codes.UNKNOWN, additional_fields=None):
         super(ProxiedRequestError, self).__init__(code, msg, errcode)
         if additional_fields is None:
-            self._additional_fields = {}
+            self._additional_fields = {}  # type: Dict
         else:
             self._additional_fields = dict(additional_fields)
 
diff --git a/synapse/api/room_versions.py b/synapse/api/room_versions.py
index 95292b7de..c6f50fd7b 100644
--- a/synapse/api/room_versions.py
+++ b/synapse/api/room_versions.py
@@ -12,6 +12,9 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
+from typing import Dict
+
 import attr
 
 
@@ -102,4 +105,4 @@ KNOWN_ROOM_VERSIONS = {
         RoomVersions.V4,
         RoomVersions.V5,
     )
-}  # type: dict[str, RoomVersion]
+}  # type: Dict[str, RoomVersion]
diff --git a/synapse/app/_base.py b/synapse/app/_base.py
index c30fdeee9..2ac7d5c06 100644
--- a/synapse/app/_base.py
+++ b/synapse/app/_base.py
@@ -263,7 +263,9 @@ def start(hs, listeners=None):
         refresh_certificate(hs)
 
         # Start the tracer
-        synapse.logging.opentracing.init_tracer(hs.config)
+        synapse.logging.opentracing.init_tracer(  # type: ignore[attr-defined] # noqa
+            hs.config
+        )
 
         # It is now safe to start your Synapse.
         hs.start_listening(listeners)
diff --git a/synapse/config/appservice.py b/synapse/config/appservice.py
index 8387ff680..28d36b1bc 100644
--- a/synapse/config/appservice.py
+++ b/synapse/config/appservice.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import logging
+from typing import Dict
 
 from six import string_types
 from six.moves.urllib import parse as urlparse
@@ -56,8 +57,8 @@ def load_appservices(hostname, config_files):
         return []
 
     # Dicts of value -> filename
-    seen_as_tokens = {}
-    seen_ids = {}
+    seen_as_tokens = {}  # type: Dict[str, str]
+    seen_ids = {}  # type: Dict[str, str]
 
     appservices = []
 
diff --git a/synapse/config/consent_config.py b/synapse/config/consent_config.py
index 94916f3a4..48976e17b 100644
--- a/synapse/config/consent_config.py
+++ b/synapse/config/consent_config.py
@@ -73,8 +73,8 @@ DEFAULT_CONFIG = """\
 
 
 class ConsentConfig(Config):
-    def __init__(self):
-        super(ConsentConfig, self).__init__()
+    def __init__(self, *args):
+        super(ConsentConfig, self).__init__(*args)
 
         self.user_consent_version = None
         self.user_consent_template_dir = None
diff --git a/synapse/config/password_auth_providers.py b/synapse/config/password_auth_providers.py
index 788c39c9f..c50e24439 100644
--- a/synapse/config/password_auth_providers.py
+++ b/synapse/config/password_auth_providers.py
@@ -13,6 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from typing import Any, List
+
 from synapse.util.module_loader import load_module
 
 from ._base import Config
@@ -22,7 +24,7 @@ LDAP_PROVIDER = "ldap_auth_provider.LdapAuthProvider"
 
 class PasswordAuthProviderConfig(Config):
     def read_config(self, config, **kwargs):
-        self.password_providers = []
+        self.password_providers = []  # type: List[Any]
         providers = []
 
         # We want to be backwards compatible with the old `ldap_config`
diff --git a/synapse/config/repository.py b/synapse/config/repository.py
index 52e014608..14740891f 100644
--- a/synapse/config/repository.py
+++ b/synapse/config/repository.py
@@ -15,6 +15,7 @@
 
 import os
 from collections import namedtuple
+from typing import Dict, List
 
 from synapse.python_dependencies import DependencyException, check_requirements
 from synapse.util.module_loader import load_module
@@ -61,7 +62,7 @@ def parse_thumbnail_requirements(thumbnail_sizes):
         Dictionary mapping from media type string to list of
         ThumbnailRequirement tuples.
     """
-    requirements = {}
+    requirements = {}  # type: Dict[str, List]
     for size in thumbnail_sizes:
         width = size["width"]
         height = size["height"]
@@ -130,7 +131,7 @@ class ContentRepositoryConfig(Config):
         #
         # We don't create the storage providers here as not all workers need
         # them to be started.
-        self.media_storage_providers = []
+        self.media_storage_providers = []  # type: List[tuple]
 
         for provider_config in storage_providers:
             # We special case the module "file_system" so as not to need to
diff --git a/synapse/config/server.py b/synapse/config/server.py
index 536ee7f29..709bd387e 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -19,6 +19,7 @@ import logging
 import os.path
 import re
 from textwrap import indent
+from typing import List
 
 import attr
 import yaml
@@ -243,7 +244,7 @@ class ServerConfig(Config):
         # events with profile information that differ from the target's global profile.
         self.allow_per_room_profiles = config.get("allow_per_room_profiles", True)
 
-        self.listeners = []
+        self.listeners = []  # type: List[dict]
         for listener in config.get("listeners", []):
             if not isinstance(listener.get("port", None), int):
                 raise ConfigError(
@@ -287,7 +288,10 @@ class ServerConfig(Config):
                 validator=attr.validators.instance_of(bool), default=False
             )
             complexity = attr.ib(
-                validator=attr.validators.instance_of((int, float)), default=1.0
+                validator=attr.validators.instance_of(
+                    (float, int)  # type: ignore[arg-type] # noqa
+                ),
+                default=1.0,
             )
             complexity_error = attr.ib(
                 validator=attr.validators.instance_of(str),
@@ -366,7 +370,7 @@ class ServerConfig(Config):
             "cleanup_extremities_with_dummy_events", True
         )
 
-    def has_tls_listener(self):
+    def has_tls_listener(self) -> bool:
         return any(l["tls"] for l in self.listeners)
 
     def generate_config_section(
diff --git a/synapse/config/server_notices_config.py b/synapse/config/server_notices_config.py
index eaac3d73b..6d4285ef9 100644
--- a/synapse/config/server_notices_config.py
+++ b/synapse/config/server_notices_config.py
@@ -59,8 +59,8 @@ class ServerNoticesConfig(Config):
             None if server notices are not enabled.
     """
 
-    def __init__(self):
-        super(ServerNoticesConfig, self).__init__()
+    def __init__(self, *args):
+        super(ServerNoticesConfig, self).__init__(*args)
         self.server_notices_mxid = None
         self.server_notices_mxid_display_name = None
         self.server_notices_mxid_avatar_url = None
diff --git a/synapse/logging/opentracing.py b/synapse/logging/opentracing.py
index 308a27213..cd1ff6a51 100644
--- a/synapse/logging/opentracing.py
+++ b/synapse/logging/opentracing.py
@@ -170,6 +170,7 @@ import inspect
 import logging
 import re
 from functools import wraps
+from typing import Dict
 
 from canonicaljson import json
 
@@ -547,7 +548,7 @@ def inject_active_span_twisted_headers(headers, destination, check_destination=T
         return
 
     span = opentracing.tracer.active_span
-    carrier = {}
+    carrier = {}  # type: Dict[str, str]
     opentracing.tracer.inject(span, opentracing.Format.HTTP_HEADERS, carrier)
 
     for key, value in carrier.items():
@@ -584,7 +585,7 @@ def inject_active_span_byte_dict(headers, destination, check_destination=True):
 
     span = opentracing.tracer.active_span
 
-    carrier = {}
+    carrier = {}  # type: Dict[str, str]
     opentracing.tracer.inject(span, opentracing.Format.HTTP_HEADERS, carrier)
 
     for key, value in carrier.items():
@@ -639,7 +640,7 @@ def get_active_span_text_map(destination=None):
     if destination and not whitelisted_homeserver(destination):
         return {}
 
-    carrier = {}
+    carrier = {}  # type: Dict[str, str]
     opentracing.tracer.inject(
         opentracing.tracer.active_span, opentracing.Format.TEXT_MAP, carrier
     )
@@ -653,7 +654,7 @@ def active_span_context_as_string():
     Returns:
         The active span context encoded as a string.
     """
-    carrier = {}
+    carrier = {}  # type: Dict[str, str]
     if opentracing:
         opentracing.tracer.inject(
             opentracing.tracer.active_span, opentracing.Format.TEXT_MAP, carrier
diff --git a/synapse/logging/utils.py b/synapse/logging/utils.py
index 7df0fa608..6073fc272 100644
--- a/synapse/logging/utils.py
+++ b/synapse/logging/utils.py
@@ -119,7 +119,11 @@ def trace_function(f):
         logger = logging.getLogger(name)
         level = logging.DEBUG
 
-        s = inspect.currentframe().f_back
+        frame = inspect.currentframe()
+        if frame is None:
+            raise Exception("Can't get current frame!")
+
+        s = frame.f_back
 
         to_print = [
             "\t%s:%s %s. Args: args=%s, kwargs=%s"
@@ -144,7 +148,7 @@ def trace_function(f):
             pathname=pathname,
             lineno=lineno,
             msg=msg,
-            args=None,
+            args=tuple(),
             exc_info=None,
         )
 
@@ -157,7 +161,12 @@ def trace_function(f):
 
 
 def get_previous_frames():
-    s = inspect.currentframe().f_back.f_back
+
+    frame = inspect.currentframe()
+    if frame is None:
+        raise Exception("Can't get current frame!")
+
+    s = frame.f_back.f_back
     to_return = []
     while s:
         if s.f_globals["__name__"].startswith("synapse"):
@@ -174,7 +183,10 @@ def get_previous_frames():
 
 
 def get_previous_frame(ignore=[]):
-    s = inspect.currentframe().f_back.f_back
+    frame = inspect.currentframe()
+    if frame is None:
+        raise Exception("Can't get current frame!")
+    s = frame.f_back.f_back
 
     while s:
         if s.f_globals["__name__"].startswith("synapse"):
diff --git a/synapse/metrics/__init__.py b/synapse/metrics/__init__.py
index bec3b1339..0b45e1f52 100644
--- a/synapse/metrics/__init__.py
+++ b/synapse/metrics/__init__.py
@@ -125,7 +125,7 @@ class InFlightGauge(object):
         )
 
         # Counts number of in flight blocks for a given set of label values
-        self._registrations = {}
+        self._registrations = {}  # type: Dict
 
         # Protects access to _registrations
         self._lock = threading.Lock()
@@ -226,7 +226,7 @@ class BucketCollector(object):
         # Fetch the data -- this must be synchronous!
         data = self.data_collector()
 
-        buckets = {}
+        buckets = {}  # type: Dict[float, int]
 
         res = []
         for x in data.keys():
diff --git a/synapse/metrics/_exposition.py b/synapse/metrics/_exposition.py
index 74d9c3ecd..a24810319 100644
--- a/synapse/metrics/_exposition.py
+++ b/synapse/metrics/_exposition.py
@@ -36,9 +36,9 @@ from twisted.web.resource import Resource
 try:
     from prometheus_client.samples import Sample
 except ImportError:
-    Sample = namedtuple(
+    Sample = namedtuple(  # type: ignore[no-redef] # noqa
         "Sample", ["name", "labels", "value", "timestamp", "exemplar"]
-    )  # type: ignore
+    )
 
 
 CONTENT_TYPE_LATEST = str("text/plain; version=0.0.4; charset=utf-8")
diff --git a/synapse/python_dependencies.py b/synapse/python_dependencies.py
index 0bd563edc..aa7da1c54 100644
--- a/synapse/python_dependencies.py
+++ b/synapse/python_dependencies.py
@@ -15,7 +15,7 @@
 # limitations under the License.
 
 import logging
-from typing import Set
+from typing import List, Set
 
 from pkg_resources import (
     DistributionNotFound,
@@ -73,6 +73,7 @@ REQUIREMENTS = [
     "netaddr>=0.7.18",
     "Jinja2>=2.9",
     "bleach>=1.4.3",
+    "typing-extensions>=3.7.4",
 ]
 
 CONDITIONAL_REQUIREMENTS = {
@@ -144,7 +145,11 @@ def check_requirements(for_feature=None):
             deps_needed.append(dependency)
             errors.append(
                 "Needed %s, got %s==%s"
-                % (dependency, e.dist.project_name, e.dist.version)
+                % (
+                    dependency,
+                    e.dist.project_name,  # type: ignore[attr-defined] # noqa
+                    e.dist.version,  # type: ignore[attr-defined] # noqa
+                )
             )
         except DistributionNotFound:
             deps_needed.append(dependency)
@@ -159,7 +164,7 @@ def check_requirements(for_feature=None):
     if not for_feature:
         # Check the optional dependencies are up to date. We allow them to not be
         # installed.
-        OPTS = sum(CONDITIONAL_REQUIREMENTS.values(), [])
+        OPTS = sum(CONDITIONAL_REQUIREMENTS.values(), [])  # type: List[str]
 
         for dependency in OPTS:
             try:
@@ -168,7 +173,11 @@ def check_requirements(for_feature=None):
                 deps_needed.append(dependency)
                 errors.append(
                     "Needed optional %s, got %s==%s"
-                    % (dependency, e.dist.project_name, e.dist.version)
+                    % (
+                        dependency,
+                        e.dist.project_name,  # type: ignore[attr-defined] # noqa
+                        e.dist.version,  # type: ignore[attr-defined] # noqa
+                    )
                 )
             except DistributionNotFound:
                 # If it's not found, we don't care
diff --git a/synapse/types.py b/synapse/types.py
index 51eadb6ad..8f79797f1 100644
--- a/synapse/types.py
+++ b/synapse/types.py
@@ -318,6 +318,7 @@ class StreamToken(
     )
 ):
     _SEPARATOR = "_"
+    START = None  # type: StreamToken
 
     @classmethod
     def from_string(cls, string):
@@ -402,7 +403,7 @@ class RoomStreamToken(namedtuple("_StreamToken", "topological stream")):
     followed by the "stream_ordering" id of the event it comes after.
     """
 
-    __slots__ = []
+    __slots__ = []  # type: list
 
     @classmethod
     def parse(cls, string):
diff --git a/synapse/util/async_helpers.py b/synapse/util/async_helpers.py
index f1c46836b..0d3bdd88c 100644
--- a/synapse/util/async_helpers.py
+++ b/synapse/util/async_helpers.py
@@ -13,9 +13,11 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
 import collections
 import logging
 from contextlib import contextmanager
+from typing import Dict, Sequence, Set, Union
 
 from six.moves import range
 
@@ -213,7 +215,9 @@ class Linearizer(object):
         # the first element is the number of things executing, and
         # the second element is an OrderedDict, where the keys are deferreds for the
         # things blocked from executing.
-        self.key_to_defer = {}
+        self.key_to_defer = (
+            {}
+        )  # type: Dict[str, Sequence[Union[int, Dict[defer.Deferred, int]]]]
 
     def queue(self, key):
         # we avoid doing defer.inlineCallbacks here, so that cancellation works correctly.
@@ -340,10 +344,10 @@ class ReadWriteLock(object):
 
     def __init__(self):
         # Latest readers queued
-        self.key_to_current_readers = {}
+        self.key_to_current_readers = {}  # type: Dict[str, Set[defer.Deferred]]
 
         # Latest writer queued
-        self.key_to_current_writer = {}
+        self.key_to_current_writer = {}  # type: Dict[str, defer.Deferred]
 
     @defer.inlineCallbacks
     def read(self, key):
diff --git a/synapse/util/caches/__init__.py b/synapse/util/caches/__init__.py
index b50e3503f..43fd65d69 100644
--- a/synapse/util/caches/__init__.py
+++ b/synapse/util/caches/__init__.py
@@ -16,6 +16,7 @@
 
 import logging
 import os
+from typing import Dict
 
 import six
 from six.moves import intern
@@ -37,7 +38,7 @@ def get_cache_factor_for(cache_name):
 
 
 caches_by_name = {}
-collectors_by_name = {}
+collectors_by_name = {}  # type: Dict
 
 cache_size = Gauge("synapse_util_caches_cache:size", "", ["name"])
 cache_hits = Gauge("synapse_util_caches_cache:hits", "", ["name"])
diff --git a/synapse/util/caches/descriptors.py b/synapse/util/caches/descriptors.py
index 43f66ec4b..5ac2530a6 100644
--- a/synapse/util/caches/descriptors.py
+++ b/synapse/util/caches/descriptors.py
@@ -18,10 +18,12 @@ import inspect
 import logging
 import threading
 from collections import namedtuple
+from typing import Any, cast
 
 from six import itervalues
 
 from prometheus_client import Gauge
+from typing_extensions import Protocol
 
 from twisted.internet import defer
 
@@ -37,6 +39,18 @@ from . import register_cache
 logger = logging.getLogger(__name__)
 
 
+class _CachedFunction(Protocol):
+    invalidate = None  # type: Any
+    invalidate_all = None  # type: Any
+    invalidate_many = None  # type: Any
+    prefill = None  # type: Any
+    cache = None  # type: Any
+    num_args = None  # type: Any
+
+    def __name__(self):
+        ...
+
+
 cache_pending_metric = Gauge(
     "synapse_util_caches_cache_pending",
     "Number of lookups currently pending for this cache",
@@ -245,7 +259,9 @@ class Cache(object):
 
 
 class _CacheDescriptorBase(object):
-    def __init__(self, orig, num_args, inlineCallbacks, cache_context=False):
+    def __init__(
+        self, orig: _CachedFunction, num_args, inlineCallbacks, cache_context=False
+    ):
         self.orig = orig
 
         if inlineCallbacks:
@@ -404,7 +420,7 @@ class CacheDescriptor(_CacheDescriptorBase):
                 return tuple(get_cache_key_gen(args, kwargs))
 
         @functools.wraps(self.orig)
-        def wrapped(*args, **kwargs):
+        def _wrapped(*args, **kwargs):
             # If we're passed a cache_context then we'll want to call its invalidate()
             # whenever we are invalidated
             invalidate_callback = kwargs.pop("on_invalidate", None)
@@ -440,6 +456,8 @@ class CacheDescriptor(_CacheDescriptorBase):
 
             return make_deferred_yieldable(observer)
 
+        wrapped = cast(_CachedFunction, _wrapped)
+
         if self.num_args == 1:
             wrapped.invalidate = lambda key: cache.invalidate(key[0])
             wrapped.prefill = lambda key, val: cache.prefill(key[0], val)
diff --git a/synapse/util/caches/treecache.py b/synapse/util/caches/treecache.py
index 9a72218d8..2ea4e4e91 100644
--- a/synapse/util/caches/treecache.py
+++ b/synapse/util/caches/treecache.py
@@ -1,3 +1,5 @@
+from typing import Dict
+
 from six import itervalues
 
 SENTINEL = object()
@@ -12,7 +14,7 @@ class TreeCache(object):
 
     def __init__(self):
         self.size = 0
-        self.root = {}
+        self.root = {}  # type: Dict
 
     def __setitem__(self, key, value):
         return self.set(key, value)
diff --git a/synapse/util/module_loader.py b/synapse/util/module_loader.py
index 7ff7eb1e4..2705cbe5f 100644
--- a/synapse/util/module_loader.py
+++ b/synapse/util/module_loader.py
@@ -54,5 +54,5 @@ def load_python_module(location: str):
     if spec is None:
         raise Exception("Unable to load module at %s" % (location,))
     mod = importlib.util.module_from_spec(spec)
-    spec.loader.exec_module(mod)
+    spec.loader.exec_module(mod)  # type: ignore
     return mod