diff --git a/flask_session/py.typed b/flask_session/py.typed
new file mode 100644
index 00000000..e69de29b
diff --git a/flask_session/sessions.py b/flask_session/sessions.py
index 40da962b..9fb0399a 100644
--- a/flask_session/sessions.py
+++ b/flask_session/sessions.py
@@ -12,7 +12,9 @@
 import os
 import sys
 import time
+import typing as t
 from datetime import datetime
+from typing import TYPE_CHECKING, Any, ClassVar, Optional, Protocol, Type
 from uuid import uuid4
 
 import pytz
@@ -22,6 +24,11 @@
 except ImportError:
     import pickle  # type: ignore[no-redef]
 
+if TYPE_CHECKING:
+    from redis import Redis
+    from flask import Flask
+    from flask.wrappers import Request
+
 from flask.sessions import SessionInterface as FlaskSessionInterface
 from flask.sessions import SessionMixin
 from itsdangerous import BadSignature, Signer, want_bytes
@@ -38,11 +45,23 @@ def total_seconds(td):
     return td.days * 60 * 60 * 24 + td.seconds
 
 
+class SessionSerializerType(Protocol):
+    def dumps(self, session_content: Any) -> bytes:
+        """serialize a session object"""
+        ...
+
+    def loads(self, serialized_session: bytes) -> Any:
+        """deserialize a session object"""
+        ...
+
+
 class ServerSideSession(CallbackDict, SessionMixin):
     """Baseclass for server-side based sessions."""
 
-    def __init__(self, initial=None, sid=None, permanent=None):
-        def on_update(self):
+    def __init__(
+        self, initial=None, sid: Optional[str] = None, permanent: Optional[bool] = None
+    ):
+        def on_update(self) -> None:
             self.modified = True
 
         CallbackDict.__init__(self, initial, on_update)
@@ -89,16 +108,25 @@ class DynamoDBSession(ServerSideSession):
 
 
 class PeeweeSession(ServerSideSession):
-    def __init__(self, initial=None, sid=None, permanent=None, ip=None):
+    def __init__(
+        self,
+        initial=None,
+        sid: Optional[str] = None,
+        permanent: Optional[bool] = None,
+        ip=None,
+    ):
         super().__init__(initial, sid, permanent)
         self.ip = ip
 
 
 class SessionInterface(FlaskSessionInterface):
-    def _generate_sid(self):
+    serializer: ClassVar[SessionSerializerType]
+    session_class: ClassVar[Type[ServerSideSession]]
+
+    def _generate_sid(self) -> str:
         return str(uuid4())
 
-    def _get_signer(self, app):
+    def _get_signer(self, app: Flask) -> Optional[Signer]:
         if not app.secret_key:
             return None
         return Signer(app.secret_key, salt="flask-session", key_derivation="hmac")
@@ -107,7 +135,7 @@ def _get_signer(self, app):
 class NullSessionInterface(SessionInterface):
     """Used to open a :class:`flask.sessions.NullSession` instance."""
 
-    def open_session(self, app, request):
+    def open_session(self, app: Flask, request: Request) -> None:
         return None
 
 
@@ -123,10 +151,16 @@ class RedisSessionInterface(SessionInterface):
     :param permanent: Whether to use permanent session or not.
     """
 
-    serializer = pickle
-    session_class = RedisSession
+    serializer: ClassVar[SessionSerializerType] = t.cast(SessionSerializerType, pickle)
+    session_class: ClassVar[Type[RedisSession]] = RedisSession
 
-    def __init__(self, redis, key_prefix, use_signer=False, permanent=True):
+    def __init__(
+        self,
+        redis: Optional[Redis],
+        key_prefix: str,
+        use_signer: bool = False,
+        permanent: bool = True,
+    ):
         if redis is None:
             from redis import Redis
 
@@ -137,7 +171,7 @@ def __init__(self, redis, key_prefix, use_signer=False, permanent=True):
         self.permanent = permanent
         self.has_same_site_capability = hasattr(self, "get_cookie_samesite")
 
-    def open_session(self, app, request):
+    def open_session(self, app: Flask, request: Request) -> Optional[SessionMixin]:
         sid = request.cookies.get(app.config["SESSION_COOKIE_NAME"])
         if not sid:
             sid = self._generate_sid()
@@ -164,7 +198,9 @@ def open_session(self, app, request):
                 return self.session_class(sid=sid, permanent=self.permanent)
         return self.session_class(sid=sid, permanent=self.permanent)
 
-    def save_session(self, app, session, response):
+    def save_session(
+        self, app: Flask, session: SessionMixin, response: Response
+    ) -> None:
         if not self.should_set_cookie(app, session):
             return
         domain = self.get_cookie_domain(app)
@@ -202,7 +238,9 @@ def save_session(self, app, session, response):
             )
 
         if self.use_signer:
-            session_id = self._get_signer(app).sign(want_bytes(session.sid))
+            session_id = t.cast(Signer, self._get_signer(app)).sign(
+                want_bytes(session.sid)
+            )
         else:
             session_id = session.sid
         response.set_cookie(