diff --git a/changelog.d/8067.misc b/changelog.d/8067.misc
new file mode 100644
index 000000000..f4404b750
--- /dev/null
+++ b/changelog.d/8067.misc
@@ -0,0 +1 @@
+Add type hints to `synapse.handlers.message` and `synapse.events.builder`.
diff --git a/mypy.ini b/mypy.ini
index a61009b19..c69cb5dc4 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -81,3 +81,6 @@ ignore_missing_imports = True
 
 [mypy-rust_python_jaeger_reporter.*]
 ignore_missing_imports = True
+
+[mypy-nacl.*]
+ignore_missing_imports = True
diff --git a/synapse/events/builder.py b/synapse/events/builder.py
index 4e179d49b..9ed24380d 100644
--- a/synapse/events/builder.py
+++ b/synapse/events/builder.py
@@ -17,6 +17,7 @@ from typing import Optional
 import attr
 from nacl.signing import SigningKey
 
+from synapse.api.auth import Auth
 from synapse.api.constants import MAX_DEPTH
 from synapse.api.errors import UnsupportedRoomVersionError
 from synapse.api.room_versions import (
@@ -27,6 +28,8 @@ from synapse.api.room_versions import (
 )
 from synapse.crypto.event_signing import add_hashes_and_signatures
 from synapse.events import EventBase, _EventInternalMetadata, make_event_from_dict
+from synapse.state import StateHandler
+from synapse.storage.databases.main import DataStore
 from synapse.types import EventID, JsonDict
 from synapse.util import Clock
 from synapse.util.stringutils import random_string
@@ -42,45 +45,46 @@ class EventBuilder(object):
 
     Attributes:
         room_version: Version of the target room
-        room_id (str)
-        type (str)
-        sender (str)
-        content (dict)
-        unsigned (dict)
-        internal_metadata (_EventInternalMetadata)
+        room_id
+        type
+        sender
+        content
+        unsigned
+        internal_metadata
 
-        _state (StateHandler)
-        _auth (synapse.api.Auth)
-        _store (DataStore)
-        _clock (Clock)
-        _hostname (str): The hostname of the server creating the event
+        _state
+        _auth
+        _store
+        _clock
+        _hostname: The hostname of the server creating the event
         _signing_key: The signing key to use to sign the event as the server
     """
 
-    _state = attr.ib()
-    _auth = attr.ib()
-    _store = attr.ib()
-    _clock = attr.ib()
-    _hostname = attr.ib()
-    _signing_key = attr.ib()
+    _state = attr.ib(type=StateHandler)
+    _auth = attr.ib(type=Auth)
+    _store = attr.ib(type=DataStore)
+    _clock = attr.ib(type=Clock)
+    _hostname = attr.ib(type=str)
+    _signing_key = attr.ib(type=SigningKey)
 
     room_version = attr.ib(type=RoomVersion)
 
-    room_id = attr.ib()
-    type = attr.ib()
-    sender = attr.ib()
+    room_id = attr.ib(type=str)
+    type = attr.ib(type=str)
+    sender = attr.ib(type=str)
 
-    content = attr.ib(default=attr.Factory(dict))
-    unsigned = attr.ib(default=attr.Factory(dict))
+    content = attr.ib(default=attr.Factory(dict), type=JsonDict)
+    unsigned = attr.ib(default=attr.Factory(dict), type=JsonDict)
 
     # These only exist on a subset of events, so they raise AttributeError if
     # someone tries to get them when they don't exist.
-    _state_key = attr.ib(default=None)
-    _redacts = attr.ib(default=None)
-    _origin_server_ts = attr.ib(default=None)
+    _state_key = attr.ib(default=None, type=Optional[str])
+    _redacts = attr.ib(default=None, type=Optional[str])
+    _origin_server_ts = attr.ib(default=None, type=Optional[int])
 
     internal_metadata = attr.ib(
-        default=attr.Factory(lambda: _EventInternalMetadata({}))
+        default=attr.Factory(lambda: _EventInternalMetadata({})),
+        type=_EventInternalMetadata,
     )
 
     @property
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 8ddded838..2643438e8 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -15,7 +15,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import TYPE_CHECKING, List, Optional, Tuple
+from typing import TYPE_CHECKING, Dict, List, Optional, Tuple
 
 from canonicaljson import encode_canonical_json, json
 
@@ -93,11 +93,11 @@ class MessageHandler(object):
 
     async def get_room_data(
         self,
-        user_id: str = None,
-        room_id: str = None,
-        event_type: Optional[str] = None,
-        state_key: str = "",
-        is_guest: bool = False,
+        user_id: str,
+        room_id: str,
+        event_type: str,
+        state_key: str,
+        is_guest: bool,
     ) -> dict:
         """ Get data from a room.
 
@@ -407,7 +407,7 @@ class EventCreationHandler(object):
         #
         # map from room id to time-of-last-attempt.
         #
-        self._rooms_to_exclude_from_dummy_event_insertion = {}  # type: dict[str, int]
+        self._rooms_to_exclude_from_dummy_event_insertion = {}  # type: Dict[str, int]
 
         # we need to construct a ConsentURIBuilder here, as it checks that the necessary
         # config options, but *only* if we have a configuration for which we are
@@ -707,7 +707,7 @@ class EventCreationHandler(object):
     async def create_and_send_nonmember_event(
         self,
         requester: Requester,
-        event_dict: EventBase,
+        event_dict: dict,
         ratelimit: bool = True,
         txn_id: Optional[str] = None,
     ) -> Tuple[EventBase, int]:
@@ -971,7 +971,7 @@ class EventCreationHandler(object):
             # Validate a newly added alias or newly added alt_aliases.
 
             original_alias = None
-            original_alt_aliases = set()
+            original_alt_aliases = []  # type: List[str]
 
             original_event_id = event.unsigned.get("replaces_state")
             if original_event_id:
@@ -1019,6 +1019,10 @@ class EventCreationHandler(object):
 
                 current_state_ids = await context.get_current_state_ids()
 
+                # We know this event is not an outlier, so this must be
+                # non-None.
+                assert current_state_ids is not None
+
                 state_to_include_ids = [
                     e_id
                     for k, e_id in current_state_ids.items()
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 8e409f24e..31705cdbd 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -16,7 +16,7 @@
 import abc
 import logging
 from http import HTTPStatus
-from typing import Dict, Iterable, List, Optional, Tuple, Union
+from typing import TYPE_CHECKING, Dict, Iterable, List, Optional, Tuple, Union
 
 from unpaddedbase64 import encode_base64
 
@@ -37,6 +37,10 @@ from synapse.util.distributor import user_joined_room, user_left_room
 
 from ._base import BaseHandler
 
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+
 logger = logging.getLogger(__name__)
 
 
@@ -48,7 +52,7 @@ class RoomMemberHandler(object):
 
     __metaclass__ = abc.ABCMeta
 
-    def __init__(self, hs):
+    def __init__(self, hs: "HomeServer"):
         self.hs = hs
         self.store = hs.get_datastore()
         self.auth = hs.get_auth()
@@ -207,7 +211,7 @@ class RoomMemberHandler(object):
             return duplicate.event_id, stream_id
 
         stream_id = await self.event_creation_handler.handle_new_client_event(
-            requester, event, context, extra_users=[target], ratelimit=ratelimit
+            requester, event, context, extra_users=[target], ratelimit=ratelimit,
         )
 
         prev_state_ids = await context.get_prev_state_ids()
@@ -1000,7 +1004,7 @@ class RoomMemberMasterHandler(RoomMemberHandler):
 
         check_complexity = self.hs.config.limit_remote_rooms.enabled
         if check_complexity and self.hs.config.limit_remote_rooms.admins_can_join:
-            check_complexity = not await self.hs.auth.is_server_admin(user)
+            check_complexity = not await self.auth.is_server_admin(user)
 
         if check_complexity:
             # Fetch the room complexity
diff --git a/tests/rest/client/test_retention.py b/tests/rest/client/test_retention.py
index e54ffea15..0b191d13c 100644
--- a/tests/rest/client/test_retention.py
+++ b/tests/rest/client/test_retention.py
@@ -144,7 +144,9 @@ class RetentionTestCase(unittest.HomeserverTestCase):
         # Get the create event to, later, check that we can still access it.
         message_handler = self.hs.get_message_handler()
         create_event = self.get_success(
-            message_handler.get_room_data(self.user_id, room_id, EventTypes.Create)
+            message_handler.get_room_data(
+                self.user_id, room_id, EventTypes.Create, state_key="", is_guest=False
+            )
         )
 
         # Send a first event to the room. This is the event we'll want to be purged at the
diff --git a/tox.ini b/tox.ini
index 45e129580..e5413eb11 100644
--- a/tox.ini
+++ b/tox.ini
@@ -179,6 +179,7 @@ commands = mypy \
             synapse/appservice \
             synapse/config \
             synapse/event_auth.py \
+            synapse/events/builder.py \
             synapse/events/spamcheck.py \
             synapse/federation \
             synapse/handlers/auth.py \
@@ -186,6 +187,7 @@ commands = mypy \
             synapse/handlers/directory.py \
             synapse/handlers/federation.py \
             synapse/handlers/identity.py \
+            synapse/handlers/message.py \
             synapse/handlers/oidc_handler.py \
             synapse/handlers/presence.py \
             synapse/handlers/room_member.py \