diff --git a/libp2p/network/connection/raw_connection_interface.py b/libp2p/network/connection/raw_connection_interface.py
index 1e35514b..088eaec1 100644
--- a/libp2p/network/connection/raw_connection_interface.py
+++ b/libp2p/network/connection/raw_connection_interface.py
@@ -1,3 +1,4 @@
+import asyncio
 from abc import ABC, abstractmethod
 
 
@@ -6,6 +7,13 @@ class IRawConnection(ABC):
     A Raw Connection provides a Reader and a Writer
     """
 
+    initiator: bool
+
+    # TODO: reader and writer shouldn't be exposed.
+    # Need better API for the consumers
+    reader: asyncio.StreamReader
+    writer: asyncio.StreamWriter
+
     @abstractmethod
     async def write(self, data: bytes) -> None:
         pass
@@ -13,3 +21,11 @@ class IRawConnection(ABC):
     @abstractmethod
     async def read(self) -> bytes:
         pass
+
+    @abstractmethod
+    def close(self) -> None:
+        pass
+
+    @abstractmethod
+    def next_stream_id(self) -> int:
+        pass
diff --git a/libp2p/stream_muxer/mplex/mplex.py b/libp2p/stream_muxer/mplex/mplex.py
index 08970ff2..e75d8eea 100644
--- a/libp2p/stream_muxer/mplex/mplex.py
+++ b/libp2p/stream_muxer/mplex/mplex.py
@@ -5,13 +5,36 @@ from .utils import encode_uvarint, decode_uvarint_from_stream
 from .mplex_stream import MplexStream
 from ..muxed_connection_interface import IMuxedConn
 
+from typing import TYPE_CHECKING, Tuple, Dict
+
+if TYPE_CHECKING:
+    from multiaddr import Multiaddr
+    from libp2p.security.secure_conn_interface import ISecureConn
+    from libp2p.network.connection.raw_connection_interface import IRawConnection
+    from libp2p.network.swarm import GenericProtocolHandlerFn
+    from libp2p.peer.id import ID
+    from libp2p.stream_muxer.muxed_stream_interface import IMuxedStream
+
 
 class Mplex(IMuxedConn):
     """
     reference: https://github.com/libp2p/go-mplex/blob/master/multiplex.go
     """
 
-    def __init__(self, secured_conn, generic_protocol_handler, peer_id):
+    secured_conn: "ISecureConn"
+    raw_conn: "IRawConnection"
+    initiator: bool
+    generic_protocol_handler = None
+    peer_id: "ID"
+    buffers: Dict[int, asyncio.Queue[bytes]]
+    stream_queue: asyncio.Queue[int]
+
+    def __init__(
+        self,
+        secured_conn: "ISecureConn",
+        generic_protocol_handler: "GenericProtocolHandlerFn",
+        peer_id: "ID",
+    ) -> None:
         """
         create a new muxed connection
         :param conn: an instance of raw connection
@@ -39,19 +62,20 @@ class Mplex(IMuxedConn):
         # Kick off reading
         asyncio.ensure_future(self.handle_incoming())
 
-    def close(self):
+    def close(self) -> None:
         """
         close the stream muxer and underlying raw connection
         """
         self.raw_conn.close()
 
-    def is_closed(self):
+    def is_closed(self) -> bool:
         """
         check connection is fully closed
         :return: true if successful
         """
+        raise NotImplementedError()
 
-    async def read_buffer(self, stream_id):
+    async def read_buffer(self, stream_id: int) -> bytes:
         """
         Read a message from stream_id's buffer, check raw connection for new messages
         :param stream_id: stream id of stream to read from
@@ -69,7 +93,9 @@ class Mplex(IMuxedConn):
         # Stream not created yet
         return None
 
-    async def open_stream(self, protocol_id, multi_addr):
+    async def open_stream(
+        self, protocol_id: str, multi_addr: "Multiaddr"
+    ) -> "IMuxedStream":
         """
         creates a new muxed_stream
         :param protocol_id: protocol_id of stream
@@ -82,7 +108,7 @@ class Mplex(IMuxedConn):
         await self.send_message(HeaderTags.NewStream, None, stream_id)
         return stream
 
-    async def accept_stream(self):
+    async def accept_stream(self) -> None:
         """
         accepts a muxed stream opened by the other end
         :return: the accepted stream
@@ -91,13 +117,12 @@ class Mplex(IMuxedConn):
         stream = MplexStream(stream_id, False, self)
         asyncio.ensure_future(self.generic_protocol_handler(stream))
 
-    async def send_message(self, flag: HeaderTags, data, stream_id):
+    async def send_message(self, flag: HeaderTags, data: bytes, stream_id: int) -> int:
         """
         sends a message over the connection
         :param header: header to use
         :param data: data to send in the message
         :param stream_id: stream the message is in
-        :return: True if success
         """
         # << by 3, then or with flag
         header = (stream_id << 3) | flag.value
@@ -112,7 +137,7 @@ class Mplex(IMuxedConn):
 
         return await self.write_to_stream(_bytes)
 
-    async def write_to_stream(self, _bytes):
+    async def write_to_stream(self, _bytes: bytearray) -> int:
         """
         writes a byte array to a raw connection
         :param _bytes: byte array to write
@@ -122,7 +147,7 @@ class Mplex(IMuxedConn):
         await self.raw_conn.writer.drain()
         return len(_bytes)
 
-    async def handle_incoming(self):
+    async def handle_incoming(self) -> None:
         """
         Read a message off of the raw connection and add it to the corresponding message buffer
         """
@@ -146,7 +171,7 @@ class Mplex(IMuxedConn):
             # Force context switch
             await asyncio.sleep(0)
 
-    async def read_message(self):
+    async def read_message(self) -> Tuple[int, int, bytes]:
         """
         Read a single message off of the raw connection
         :return: stream_id, flag, message contents
diff --git a/libp2p/stream_muxer/mplex/mplex_stream.py b/libp2p/stream_muxer/mplex/mplex_stream.py
index e452fbda..a2502a88 100644
--- a/libp2p/stream_muxer/mplex/mplex_stream.py
+++ b/libp2p/stream_muxer/mplex/mplex_stream.py
@@ -10,7 +10,7 @@ class MplexStream(IMuxedStream):
     reference: https://github.com/libp2p/go-mplex/blob/master/stream.go
     """
 
-    def __init__(self, stream_id, initiator, mplex_conn):
+    def __init__(self, stream_id, initiator: bool, mplex_conn):
         """
         create new MuxedStream in muxer
         :param stream_id: stream stream id