diff --git a/bumble/controller.py b/bumble/controller.py
index 382be13..9b2960a 100644
--- a/bumble/controller.py
+++ b/bumble/controller.py
@@ -15,6 +15,8 @@
 # -----------------------------------------------------------------------------
 # Imports
 # -----------------------------------------------------------------------------
+from __future__ import annotations
+
 import logging
 import asyncio
 import itertools
@@ -58,8 +60,10 @@ from bumble.hci import (
     HCI_Packet,
     HCI_Role_Change_Event,
 )
-from typing import Optional, Union, Dict
+from typing import Optional, Union, Dict, TYPE_CHECKING
 
+if TYPE_CHECKING:
+    from bumble.transport.common import TransportSink, TransportSource
 
 # -----------------------------------------------------------------------------
 # Logging
@@ -104,7 +108,7 @@ class Controller:
         self,
         name,
         host_source=None,
-        host_sink=None,
+        host_sink: Optional[TransportSink] = None,
         link=None,
         public_address: Optional[Union[bytes, str, Address]] = None,
     ):
diff --git a/bumble/device.py b/bumble/device.py
index 46ce012..fca2e7a 100644
--- a/bumble/device.py
+++ b/bumble/device.py
@@ -23,7 +23,18 @@ import asyncio
 import logging
 from contextlib import asynccontextmanager, AsyncExitStack
 from dataclasses import dataclass
-from typing import Any, Callable, ClassVar, Dict, List, Optional, Tuple, Type, Union
+from typing import (
+    Any,
+    Callable,
+    ClassVar,
+    Dict,
+    List,
+    Optional,
+    Tuple,
+    Type,
+    Union,
+    TYPE_CHECKING,
+)
 
 from .colors import color
 from .att import ATT_CID, ATT_DEFAULT_MTU, ATT_PDU
@@ -152,6 +163,9 @@ from . import sdp
 from . import l2cap
 from . import core
 
+if TYPE_CHECKING:
+    from .transport.common import TransportSource, TransportSink
+
 
 # -----------------------------------------------------------------------------
 # Logging
@@ -942,7 +956,13 @@ class Device(CompositeEventEmitter):
             pass
 
     @classmethod
-    def with_hci(cls, name, address, hci_source, hci_sink):
+    def with_hci(
+        cls,
+        name: str,
+        address: Address,
+        hci_source: TransportSource,
+        hci_sink: TransportSink,
+    ) -> Device:
         '''
         Create a Device instance with a Host configured to communicate with a controller
         through an HCI source/sink
@@ -951,18 +971,25 @@ class Device(CompositeEventEmitter):
         return cls(name=name, address=address, host=host)
 
     @classmethod
-    def from_config_file(cls, filename):
+    def from_config_file(cls, filename: str) -> Device:
         config = DeviceConfiguration()
         config.load_from_file(filename)
         return cls(config=config)
 
     @classmethod
-    def from_config_with_hci(cls, config, hci_source, hci_sink):
+    def from_config_with_hci(
+        cls,
+        config: DeviceConfiguration,
+        hci_source: TransportSource,
+        hci_sink: TransportSink,
+    ) -> Device:
         host = Host(controller_source=hci_source, controller_sink=hci_sink)
         return cls(config=config, host=host)
 
     @classmethod
-    def from_config_file_with_hci(cls, filename, hci_source, hci_sink):
+    def from_config_file_with_hci(
+        cls, filename: str, hci_source: TransportSource, hci_sink: TransportSink
+    ) -> Device:
         config = DeviceConfiguration()
         config.load_from_file(filename)
         return cls.from_config_with_hci(config, hci_source, hci_sink)
@@ -2238,9 +2265,11 @@ class Device(CompositeEventEmitter):
     def request_pairing(self, connection):
         return self.smp_manager.request_pairing(connection)
 
-    async def get_long_term_key(self, connection_handle, rand, ediv):
+    async def get_long_term_key(
+        self, connection_handle: int, rand: bytes, ediv: int
+    ) -> Optional[bytes]:
         if (connection := self.lookup_connection(connection_handle)) is None:
-            return
+            return None
 
         # Start by looking for the key in an SMP session
         ltk = self.smp_manager.get_long_term_key(connection, rand, ediv)
@@ -2260,6 +2289,7 @@ class Device(CompositeEventEmitter):
 
                 if connection.role == BT_PERIPHERAL_ROLE and keys.ltk_peripheral:
                     return keys.ltk_peripheral.value
+        return None
 
     async def get_link_key(self, address: Address) -> Optional[bytes]:
         if self.keystore is None:
diff --git a/bumble/host.py b/bumble/host.py
index 288b1b6..02caa46 100644
--- a/bumble/host.py
+++ b/bumble/host.py
@@ -21,7 +21,7 @@ import collections
 import logging
 import struct
 
-from typing import Optional
+from typing import Optional, TYPE_CHECKING, Dict, Callable, Awaitable
 
 from bumble.colors import color
 from bumble.l2cap import L2CAP_PDU
@@ -73,10 +73,14 @@ from .core import (
     BT_LE_TRANSPORT,
     ConnectionPHY,
     ConnectionParameters,
+    InvalidStateError,
 )
 from .utils import AbortableEventEmitter
 from .transport.common import TransportLostError
 
+if TYPE_CHECKING:
+    from .transport.common import TransportSink, TransportSource
+
 
 # -----------------------------------------------------------------------------
 # Logging
@@ -116,10 +120,21 @@ class Connection:
 
 # -----------------------------------------------------------------------------
 class Host(AbortableEventEmitter):
-    def __init__(self, controller_source=None, controller_sink=None):
+    connections: Dict[int, Connection]
+    acl_packet_queue: collections.deque[HCI_AclDataPacket]
+    hci_sink: TransportSink
+    long_term_key_provider: Optional[
+        Callable[[int, bytes, int], Awaitable[Optional[bytes]]]
+    ]
+    link_key_provider: Optional[Callable[[Address], Awaitable[Optional[bytes]]]]
+
+    def __init__(
+        self,
+        controller_source: Optional[TransportSource] = None,
+        controller_sink: Optional[TransportSink] = None,
+    ) -> None:
         super().__init__()
 
-        self.hci_sink = None
         self.hci_metadata = None
         self.ready = False  # True when we can accept incoming packets
         self.reset_done = False
@@ -299,7 +314,7 @@ class Host(AbortableEventEmitter):
         self.reset_done = True
 
     @property
-    def controller(self):
+    def controller(self) -> TransportSink:
         return self.hci_sink
 
     @controller.setter
@@ -308,13 +323,12 @@ class Host(AbortableEventEmitter):
         if controller:
             controller.set_packet_sink(self)
 
-    def set_packet_sink(self, sink):
+    def set_packet_sink(self, sink: TransportSink) -> None:
         self.hci_sink = sink
 
     def send_hci_packet(self, packet: HCI_Packet) -> None:
         if self.snooper:
             self.snooper.snoop(bytes(packet), Snooper.Direction.HOST_TO_CONTROLLER)
-
         self.hci_sink.on_packet(bytes(packet))
 
     async def send_command(self, command, check_result=False):
diff --git a/bumble/transport/__init__.py b/bumble/transport/__init__.py
index c722283..bc0766b 100644
--- a/bumble/transport/__init__.py
+++ b/bumble/transport/__init__.py
@@ -20,7 +20,6 @@ import logging
 import os
 
 from .common import Transport, AsyncPipeSink, SnoopingTransport
-from ..controller import Controller
 from ..snoop import create_snooper
 
 # -----------------------------------------------------------------------------
@@ -119,7 +118,8 @@ async def _open_transport(name: str) -> Transport:
     if scheme == 'file':
         from .file import open_file_transport
 
-        return await open_file_transport(spec[0] if spec else None)
+        assert spec is not None
+        return await open_file_transport(spec[0])
 
     if scheme == 'vhci':
         from .vhci import open_vhci_transport
@@ -134,12 +134,14 @@ async def _open_transport(name: str) -> Transport:
     if scheme == 'usb':
         from .usb import open_usb_transport
 
-        return await open_usb_transport(spec[0] if spec else None)
+        assert spec is not None
+        return await open_usb_transport(spec[0])
 
     if scheme == 'pyusb':
         from .pyusb import open_pyusb_transport
 
-        return await open_pyusb_transport(spec[0] if spec else None)
+        assert spec is not None
+        return await open_pyusb_transport(spec[0])
 
     if scheme == 'android-emulator':
         from .android_emulator import open_android_emulator_transport
@@ -168,6 +170,7 @@ async def open_transport_or_link(name: str) -> Transport:
 
     """
     if name.startswith('link-relay:'):
+        from ..controller import Controller
         from ..link import RemoteLink  # lazy import
 
         link = RemoteLink(name[11:])
diff --git a/bumble/transport/android_emulator.py b/bumble/transport/android_emulator.py
index b78e263..5ef0047 100644
--- a/bumble/transport/android_emulator.py
+++ b/bumble/transport/android_emulator.py
@@ -18,7 +18,7 @@
 import logging
 import grpc.aio
 
-from .common import PumpedTransport, PumpedPacketSource, PumpedPacketSink
+from .common import PumpedTransport, PumpedPacketSource, PumpedPacketSink, Transport
 
 # pylint: disable=no-name-in-module
 from .grpc_protobuf.emulated_bluetooth_pb2_grpc import EmulatedBluetoothServiceStub
@@ -33,7 +33,7 @@ logger = logging.getLogger(__name__)
 
 
 # -----------------------------------------------------------------------------
-async def open_android_emulator_transport(spec):
+async def open_android_emulator_transport(spec: str | None) -> Transport:
     '''
     Open a transport connection to an Android emulator via its gRPC interface.
     The parameter string has this syntax:
@@ -66,7 +66,7 @@ async def open_android_emulator_transport(spec):
     # Parse the parameters
     mode = 'host'
     server_host = 'localhost'
-    server_port = 8554
+    server_port = '8554'
     if spec is not None:
         params = spec.split(',')
         for param in params:
@@ -82,6 +82,7 @@ async def open_android_emulator_transport(spec):
     logger.debug(f'connecting to gRPC server at {server_address}')
     channel = grpc.aio.insecure_channel(server_address)
 
+    service: EmulatedBluetoothServiceStub | VhciForwardingServiceStub
     if mode == 'host':
         # Connect as a host
         service = EmulatedBluetoothServiceStub(channel)
diff --git a/bumble/transport/android_netsim.py b/bumble/transport/android_netsim.py
index 99ebf87..76a7385 100644
--- a/bumble/transport/android_netsim.py
+++ b/bumble/transport/android_netsim.py
@@ -121,7 +121,9 @@ def publish_grpc_port(grpc_port) -> bool:
 
 
 # -----------------------------------------------------------------------------
-async def open_android_netsim_controller_transport(server_host, server_port):
+async def open_android_netsim_controller_transport(
+    server_host: str | None, server_port: int
+) -> Transport:
     if not server_port:
         raise ValueError('invalid port')
     if server_host == '_' or not server_host:
diff --git a/bumble/transport/common.py b/bumble/transport/common.py
index c7be3ad..5d5bdf1 100644
--- a/bumble/transport/common.py
+++ b/bumble/transport/common.py
@@ -20,11 +20,12 @@ import contextlib
 import struct
 import asyncio
 import logging
-from typing import ContextManager
+import io
+from typing import ContextManager, Tuple, Optional, Protocol, Dict
 
-from .. import hci
-from ..colors import color
-from ..snoop import Snooper
+from bumble import hci
+from bumble.colors import color
+from bumble.snoop import Snooper
 
 
 # -----------------------------------------------------------------------------
@@ -36,7 +37,7 @@ logger = logging.getLogger(__name__)
 # Information needed to parse HCI packets with a generic parser:
 # For each packet type, the info represents:
 # (length-size, length-offset, unpack-type)
-HCI_PACKET_INFO = {
+HCI_PACKET_INFO: Dict[int, Tuple[int, int, str]] = {
     hci.HCI_COMMAND_PACKET: (1, 2, 'B'),
     hci.HCI_ACL_DATA_PACKET: (2, 2, 'H'),
     hci.HCI_SYNCHRONOUS_DATA_PACKET: (1, 2, 'B'),
@@ -44,6 +45,8 @@ HCI_PACKET_INFO = {
 }
 
 
+# -----------------------------------------------------------------------------
+# Errors
 # -----------------------------------------------------------------------------
 class TransportLostError(Exception):
     """
@@ -51,24 +54,34 @@ class TransportLostError(Exception):
     """
 
 
+# -----------------------------------------------------------------------------
+# Typing Protocols
+# -----------------------------------------------------------------------------
+class TransportSink(Protocol):
+    def on_packet(self, packet: bytes) -> None:
+        ...
+
+
+class TransportSource(Protocol):
+    def set_packet_sink(self, sink: TransportSink) -> None:
+        ...
+
+
 # -----------------------------------------------------------------------------
 class PacketPump:
     """
     Pump HCI packets from a reader to a sink.
     """
 
-    def __init__(self, reader, sink):
+    def __init__(self, reader: AsyncPacketReader, sink: TransportSink) -> None:
         self.reader = reader
         self.sink = sink
 
-    async def run(self):
+    async def run(self) -> None:
         while True:
             try:
-                # Get a packet from the source
-                packet = hci.HCI_Packet.from_bytes(await self.reader.next_packet())
-
                 # Deliver the packet to the sink
-                self.sink.on_packet(packet)
+                self.sink.on_packet(await self.reader.next_packet())
             except Exception as error:
                 logger.warning(f'!!! {error}')
 
@@ -86,18 +99,22 @@ class PacketParser:
     NEED_LENGTH = 1
     NEED_BODY = 2
 
-    def __init__(self, sink=None):
+    sink: Optional[TransportSink]
+    extended_packet_info: Dict[int, Tuple[int, int, str]]
+    packet_info: Optional[Tuple[int, int, str]] = None
+
+    def __init__(self, sink: Optional[TransportSink] = None) -> None:
         self.sink = sink
         self.extended_packet_info = {}
         self.reset()
 
-    def reset(self):
+    def reset(self) -> None:
         self.state = PacketParser.NEED_TYPE
         self.bytes_needed = 1
         self.packet = bytearray()
         self.packet_info = None
 
-    def feed_data(self, data):
+    def feed_data(self, data: bytes) -> None:
         data_offset = 0
         data_left = len(data)
         while data_left and self.bytes_needed:
@@ -118,6 +135,7 @@ class PacketParser:
                     self.state = PacketParser.NEED_LENGTH
                     self.bytes_needed = self.packet_info[0] + self.packet_info[1]
                 elif self.state == PacketParser.NEED_LENGTH:
+                    assert self.packet_info is not None
                     body_length = struct.unpack_from(
                         self.packet_info[2], self.packet, 1 + self.packet_info[1]
                     )[0]
@@ -135,7 +153,7 @@ class PacketParser:
                             )
                     self.reset()
 
-    def set_packet_sink(self, sink):
+    def set_packet_sink(self, sink: TransportSink) -> None:
         self.sink = sink
 
 
@@ -145,10 +163,10 @@ class PacketReader:
     Reader that reads HCI packets from a sync source.
     """
 
-    def __init__(self, source):
+    def __init__(self, source: io.BufferedReader) -> None:
         self.source = source
 
-    def next_packet(self):
+    def next_packet(self) -> Optional[bytes]:
         # Get the packet type
         packet_type = self.source.read(1)
         if len(packet_type) != 1:
@@ -157,7 +175,7 @@ class PacketReader:
         # Get the packet info based on its type
         packet_info = HCI_PACKET_INFO.get(packet_type[0])
         if packet_info is None:
-            raise ValueError(f'invalid packet type {packet_type} found')
+            raise ValueError(f'invalid packet type {packet_type[0]} found')
 
         # Read the header (that includes the length)
         header_size = packet_info[0] + packet_info[1]
@@ -180,17 +198,17 @@ class AsyncPacketReader:
     Reader that reads HCI packets from an async source.
     """
 
-    def __init__(self, source):
+    def __init__(self, source: asyncio.StreamReader) -> None:
         self.source = source
 
-    async def next_packet(self):
+    async def next_packet(self) -> bytes:
         # Get the packet type
         packet_type = await self.source.readexactly(1)
 
         # Get the packet info based on its type
         packet_info = HCI_PACKET_INFO.get(packet_type[0])
         if packet_info is None:
-            raise ValueError(f'invalid packet type {packet_type} found')
+            raise ValueError(f'invalid packet type {packet_type[0]} found')
 
         # Read the header (that includes the length)
         header_size = packet_info[0] + packet_info[1]
@@ -209,11 +227,11 @@ class AsyncPipeSink:
     Sink that forwards packets asynchronously to another sink.
     """
 
-    def __init__(self, sink):
+    def __init__(self, sink: TransportSink) -> None:
         self.sink = sink
         self.loop = asyncio.get_running_loop()
 
-    def on_packet(self, packet):
+    def on_packet(self, packet: bytes) -> None:
         self.loop.call_soon(self.sink.on_packet, packet)
 
 
@@ -223,50 +241,48 @@ class ParserSource:
     Base class designed to be subclassed by transport-specific source classes
     """
 
-    terminated: asyncio.Future
+    terminated: asyncio.Future[None]
     parser: PacketParser
 
-    def __init__(self):
+    def __init__(self) -> None:
         self.parser = PacketParser()
         self.terminated = asyncio.get_running_loop().create_future()
 
-    def set_packet_sink(self, sink):
+    def set_packet_sink(self, sink: TransportSink) -> None:
         self.parser.set_packet_sink(sink)
 
-    def on_transport_lost(self):
+    def on_transport_lost(self) -> None:
         self.terminated.set_result(None)
         if self.parser.sink:
-            try:
+            if hasattr(self.parser.sink, 'on_transport_lost'):
                 self.parser.sink.on_transport_lost()
-            except AttributeError:
-                pass
 
-    async def wait_for_termination(self):
+    async def wait_for_termination(self) -> None:
         """
         Convenience method for backward compatibility. Prefer using the `terminated`
         attribute instead.
         """
         return await self.terminated
 
-    def close(self):
+    def close(self) -> None:
         pass
 
 
 # -----------------------------------------------------------------------------
 class StreamPacketSource(asyncio.Protocol, ParserSource):
-    def data_received(self, data):
+    def data_received(self, data: bytes) -> None:
         self.parser.feed_data(data)
 
 
 # -----------------------------------------------------------------------------
 class StreamPacketSink:
-    def __init__(self, transport):
+    def __init__(self, transport: asyncio.WriteTransport) -> None:
         self.transport = transport
 
-    def on_packet(self, packet):
+    def on_packet(self, packet: bytes) -> None:
         self.transport.write(packet)
 
-    def close(self):
+    def close(self) -> None:
         self.transport.close()
 
 
@@ -286,7 +302,7 @@ class Transport:
         ...
     """
 
-    def __init__(self, source, sink):
+    def __init__(self, source: TransportSource, sink: TransportSink) -> None:
         self.source = source
         self.sink = sink
 
@@ -300,19 +316,23 @@ class Transport:
         return iter((self.source, self.sink))
 
     async def close(self) -> None:
-        self.source.close()
-        self.sink.close()
+        if hasattr(self.source, 'close'):
+            self.source.close()
+        if hasattr(self.sink, 'close'):
+            self.sink.close()
 
 
 # -----------------------------------------------------------------------------
 class PumpedPacketSource(ParserSource):
-    def __init__(self, receive):
+    pump_task: Optional[asyncio.Task[None]]
+
+    def __init__(self, receive) -> None:
         super().__init__()
         self.receive_function = receive
         self.pump_task = None
 
-    def start(self):
-        async def pump_packets():
+    def start(self) -> None:
+        async def pump_packets() -> None:
             while True:
                 try:
                     packet = await self.receive_function()
@@ -322,12 +342,12 @@ class PumpedPacketSource(ParserSource):
                     break
                 except Exception as error:
                     logger.warning(f'exception while waiting for packet: {error}')
-                    self.terminated.set_result(error)
+                    self.terminated.set_exception(error)
                     break
 
         self.pump_task = asyncio.create_task(pump_packets())
 
-    def close(self):
+    def close(self) -> None:
         if self.pump_task:
             self.pump_task.cancel()
 
@@ -339,7 +359,7 @@ class PumpedPacketSink:
         self.packet_queue = asyncio.Queue()
         self.pump_task = None
 
-    def on_packet(self, packet):
+    def on_packet(self, packet: bytes) -> None:
         self.packet_queue.put_nowait(packet)
 
     def start(self):
@@ -364,15 +384,23 @@ class PumpedPacketSink:
 
 # -----------------------------------------------------------------------------
 class PumpedTransport(Transport):
-    def __init__(self, source, sink, close_function):
+    source: PumpedPacketSource
+    sink: PumpedPacketSink
+
+    def __init__(
+        self,
+        source: PumpedPacketSource,
+        sink: PumpedPacketSink,
+        close_function,
+    ) -> None:
         super().__init__(source, sink)
         self.close_function = close_function
 
-    def start(self):
+    def start(self) -> None:
         self.source.start()
         self.sink.start()
 
-    async def close(self):
+    async def close(self) -> None:
         await super().close()
         await self.close_function()
 
@@ -397,31 +425,37 @@ class SnoopingTransport(Transport):
         raise RuntimeError('unexpected code path')  # Satisfy the type checker
 
     class Source:
-        def __init__(self, source, snooper):
+        sink: TransportSink
+
+        def __init__(self, source: TransportSource, snooper: Snooper):
             self.source = source
             self.snooper = snooper
-            self.sink = None
 
-        def set_packet_sink(self, sink):
+        def set_packet_sink(self, sink: TransportSink) -> None:
             self.sink = sink
             self.source.set_packet_sink(self)
 
-        def on_packet(self, packet):
+        def on_packet(self, packet: bytes) -> None:
             self.snooper.snoop(packet, Snooper.Direction.CONTROLLER_TO_HOST)
             if self.sink:
                 self.sink.on_packet(packet)
 
     class Sink:
-        def __init__(self, sink, snooper):
+        def __init__(self, sink: TransportSink, snooper: Snooper) -> None:
             self.sink = sink
             self.snooper = snooper
 
-        def on_packet(self, packet):
+        def on_packet(self, packet: bytes) -> None:
             self.snooper.snoop(packet, Snooper.Direction.HOST_TO_CONTROLLER)
             if self.sink:
                 self.sink.on_packet(packet)
 
-    def __init__(self, transport, snooper, close_snooper=None):
+    def __init__(
+        self,
+        transport: Transport,
+        snooper: Snooper,
+        close_snooper=None,
+    ) -> None:
         super().__init__(
             self.Source(transport.source, snooper), self.Sink(transport.sink, snooper)
         )
diff --git a/bumble/transport/file.py b/bumble/transport/file.py
index 9c073d2..dee1c23 100644
--- a/bumble/transport/file.py
+++ b/bumble/transport/file.py
@@ -28,7 +28,7 @@ logger = logging.getLogger(__name__)
 
 
 # -----------------------------------------------------------------------------
-async def open_file_transport(spec):
+async def open_file_transport(spec: str) -> Transport:
     '''
     Open a File transport (typically not for a real file, but for a PTY or other unix
     virtual files).
diff --git a/bumble/transport/hci_socket.py b/bumble/transport/hci_socket.py
index 4e1ad99..9891c5b 100644
--- a/bumble/transport/hci_socket.py
+++ b/bumble/transport/hci_socket.py
@@ -33,7 +33,7 @@ logger = logging.getLogger(__name__)
 
 
 # -----------------------------------------------------------------------------
-async def open_hci_socket_transport(spec):
+async def open_hci_socket_transport(spec: str | None) -> Transport:
     '''
     Open an HCI Socket (only available on some platforms).
     The parameter string is either empty (to use the first/default Bluetooth adapter)
@@ -47,7 +47,7 @@ async def open_hci_socket_transport(spec):
         hci_socket = socket.socket(
             socket.AF_BLUETOOTH,
             socket.SOCK_RAW | socket.SOCK_NONBLOCK,
-            socket.BTPROTO_HCI,
+            socket.BTPROTO_HCI,  # type: ignore
         )
     except AttributeError as error:
         # Not supported on this platform
diff --git a/bumble/transport/pty.py b/bumble/transport/pty.py
index e6e2ab5..7765b09 100644
--- a/bumble/transport/pty.py
+++ b/bumble/transport/pty.py
@@ -32,7 +32,7 @@ logger = logging.getLogger(__name__)
 
 
 # -----------------------------------------------------------------------------
-async def open_pty_transport(spec):
+async def open_pty_transport(spec: str | None) -> Transport:
     '''
     Open a PTY transport.
     The parameter string may be empty, or a path name where a symbolic link
diff --git a/bumble/transport/pyusb.py b/bumble/transport/pyusb.py
index 8ad8598..5e686d1 100644
--- a/bumble/transport/pyusb.py
+++ b/bumble/transport/pyusb.py
@@ -35,7 +35,7 @@ logger = logging.getLogger(__name__)
 
 
 # -----------------------------------------------------------------------------
-async def open_pyusb_transport(spec):
+async def open_pyusb_transport(spec: str) -> Transport:
     '''
     Open a USB transport. [Implementation based on PyUSB]
     The parameter string has this syntax:
diff --git a/bumble/transport/serial.py b/bumble/transport/serial.py
index c83b605..c48cdc6 100644
--- a/bumble/transport/serial.py
+++ b/bumble/transport/serial.py
@@ -28,7 +28,7 @@ logger = logging.getLogger(__name__)
 
 
 # -----------------------------------------------------------------------------
-async def open_serial_transport(spec):
+async def open_serial_transport(spec: str) -> Transport:
     '''
     Open a serial port transport.
     The parameter string has this syntax:
diff --git a/bumble/transport/tcp_client.py b/bumble/transport/tcp_client.py
index 456a19a..4fb268a 100644
--- a/bumble/transport/tcp_client.py
+++ b/bumble/transport/tcp_client.py
@@ -27,7 +27,7 @@ logger = logging.getLogger(__name__)
 
 
 # -----------------------------------------------------------------------------
-async def open_tcp_client_transport(spec):
+async def open_tcp_client_transport(spec: str) -> Transport:
     '''
     Open a TCP client transport.
     The parameter string has this syntax:
diff --git a/bumble/transport/tcp_server.py b/bumble/transport/tcp_server.py
index 11b0453..77d0304 100644
--- a/bumble/transport/tcp_server.py
+++ b/bumble/transport/tcp_server.py
@@ -15,6 +15,7 @@
 # -----------------------------------------------------------------------------
 # Imports
 # -----------------------------------------------------------------------------
+from __future__ import annotations
 import asyncio
 import logging
 
@@ -27,7 +28,7 @@ logger = logging.getLogger(__name__)
 
 
 # -----------------------------------------------------------------------------
-async def open_tcp_server_transport(spec):
+async def open_tcp_server_transport(spec: str) -> Transport:
     '''
     Open a TCP server transport.
     The parameter string has this syntax:
@@ -42,7 +43,7 @@ async def open_tcp_server_transport(spec):
         async def close(self):
             await super().close()
 
-    class TcpServerProtocol:
+    class TcpServerProtocol(asyncio.BaseProtocol):
         def __init__(self, packet_source, packet_sink):
             self.packet_source = packet_source
             self.packet_sink = packet_sink
diff --git a/bumble/transport/udp.py b/bumble/transport/udp.py
index e5e26fa..faa9bf0 100644
--- a/bumble/transport/udp.py
+++ b/bumble/transport/udp.py
@@ -27,7 +27,7 @@ logger = logging.getLogger(__name__)
 
 
 # -----------------------------------------------------------------------------
-async def open_udp_transport(spec):
+async def open_udp_transport(spec: str) -> Transport:
     '''
     Open a UDP transport.
     The parameter string has this syntax:
diff --git a/bumble/transport/usb.py b/bumble/transport/usb.py
index 13cad60..ccc82c1 100644
--- a/bumble/transport/usb.py
+++ b/bumble/transport/usb.py
@@ -60,7 +60,7 @@ def load_libusb():
             usb1.loadLibrary(libusb_dll)
 
 
-async def open_usb_transport(spec):
+async def open_usb_transport(spec: str) -> Transport:
     '''
     Open a USB transport.
     The moniker string has this syntax:
diff --git a/bumble/transport/vhci.py b/bumble/transport/vhci.py
index ec61ab4..5795840 100644
--- a/bumble/transport/vhci.py
+++ b/bumble/transport/vhci.py
@@ -17,6 +17,7 @@
 # -----------------------------------------------------------------------------
 import logging
 
+from .common import Transport
 from .file import open_file_transport
 
 # -----------------------------------------------------------------------------
@@ -26,7 +27,7 @@ logger = logging.getLogger(__name__)
 
 
 # -----------------------------------------------------------------------------
-async def open_vhci_transport(spec):
+async def open_vhci_transport(spec: str | None) -> Transport:
     '''
     Open a VHCI transport (only available on some platforms).
     The parameter string is either empty (to use the default VHCI device
@@ -42,15 +43,15 @@ async def open_vhci_transport(spec):
     # Override the source's `data_received` method so that we can
     # filter out the vendor packet that is received just after the
     # initial open
-    def vhci_data_received(data):
+    def vhci_data_received(data: bytes) -> None:
         if len(data) > 0 and data[0] == HCI_VENDOR_PKT:
             if len(data) == 4:
                 hci_index = data[2] << 8 | data[3]
                 logger.info(f'HCI index {hci_index}')
         else:
-            transport.source.parser.feed_data(data)
+            transport.source.parser.feed_data(data)  # type: ignore
 
-    transport.source.data_received = vhci_data_received
+    transport.source.data_received = vhci_data_received  # type: ignore
 
     # Write the initial config
     transport.sink.on_packet(bytes([HCI_VENDOR_PKT, HCI_BREDR]))
diff --git a/bumble/transport/ws_client.py b/bumble/transport/ws_client.py
index 85f6e88..facd1c9 100644
--- a/bumble/transport/ws_client.py
+++ b/bumble/transport/ws_client.py
@@ -16,9 +16,9 @@
 # Imports
 # -----------------------------------------------------------------------------
 import logging
-import websockets
+import websockets.client
 
-from .common import PumpedPacketSource, PumpedPacketSink, PumpedTransport
+from .common import PumpedPacketSource, PumpedPacketSink, PumpedTransport, Transport
 
 # -----------------------------------------------------------------------------
 # Logging
@@ -27,7 +27,7 @@ logger = logging.getLogger(__name__)
 
 
 # -----------------------------------------------------------------------------
-async def open_ws_client_transport(spec):
+async def open_ws_client_transport(spec: str) -> Transport:
     '''
     Open a WebSocket client transport.
     The parameter string has this syntax:
@@ -38,7 +38,7 @@ async def open_ws_client_transport(spec):
 
     remote_host, remote_port = spec.split(':')
     uri = f'ws://{remote_host}:{remote_port}'
-    websocket = await websockets.connect(uri)
+    websocket = await websockets.client.connect(uri)
 
     transport = PumpedTransport(
         PumpedPacketSource(websocket.recv),
diff --git a/bumble/transport/ws_server.py b/bumble/transport/ws_server.py
index ddebef2..3c72c36 100644
--- a/bumble/transport/ws_server.py
+++ b/bumble/transport/ws_server.py
@@ -15,7 +15,6 @@
 # -----------------------------------------------------------------------------
 # Imports
 # -----------------------------------------------------------------------------
-import asyncio
 import logging
 import websockets
 
@@ -28,7 +27,7 @@ logger = logging.getLogger(__name__)
 
 
 # -----------------------------------------------------------------------------
-async def open_ws_server_transport(spec):
+async def open_ws_server_transport(spec: str) -> Transport:
     '''
     Open a WebSocket server transport.
     The parameter string has this syntax: