diff --git a/bleak/backends/characteristic.py b/bleak/backends/characteristic.py
index c73d58b1..b6fb2594 100644
--- a/bleak/backends/characteristic.py
+++ b/bleak/backends/characteristic.py
@@ -107,7 +107,7 @@ def get_descriptor(
         raise NotImplementedError()
 
     @abc.abstractmethod
-    def add_descriptor(self, descriptor: BleakGATTDescriptor):
+    def add_descriptor(self, descriptor: BleakGATTDescriptor) -> None:
         """Add a :py:class:`~BleakGATTDescriptor` to the characteristic.
 
         Should not be used by end user, but rather by `bleak` itself.
diff --git a/bleak/backends/scanner.py b/bleak/backends/scanner.py
index 60396fa7..591cd152 100644
--- a/bleak/backends/scanner.py
+++ b/bleak/backends/scanner.py
@@ -180,7 +180,7 @@ def register_detection_callback(
 
         if inspect.iscoroutinefunction(callback):
 
-            def detection_callback(s, d):
+            def detection_callback(s: BLEDevice, d: AdvertisementData) -> None:
                 task = asyncio.create_task(callback(s, d))
                 _background_tasks.add(task)
                 task.add_done_callback(_background_tasks.discard)
@@ -192,7 +192,7 @@ def detection_callback(s, d):
 
         self._ad_callbacks[token] = detection_callback
 
-        def remove():
+        def remove() -> None:
             self._ad_callbacks.pop(token, None)
 
         return remove
diff --git a/bleak/backends/service.py b/bleak/backends/service.py
index 305e8bd2..2d587c58 100644
--- a/bleak/backends/service.py
+++ b/bleak/backends/service.py
@@ -7,7 +7,7 @@
 """
 import abc
 import logging
-from typing import Dict, Iterator, List, Optional, Union
+from typing import Any, Dict, Iterator, List, Optional, Union
 from uuid import UUID
 
 from ..exc import BleakError
@@ -21,10 +21,10 @@
 class BleakGATTService(abc.ABC):
     """Interface for the Bleak representation of a GATT Service."""
 
-    def __init__(self, obj):
+    def __init__(self, obj: Any) -> None:
         self.obj = obj
 
-    def __str__(self):
+    def __str__(self) -> str:
         return f"{self.uuid} (Handle: {self.handle}): {self.description}"
 
     @property
@@ -51,7 +51,7 @@ def characteristics(self) -> List[BleakGATTCharacteristic]:
         raise NotImplementedError()
 
     @abc.abstractmethod
-    def add_characteristic(self, characteristic: BleakGATTCharacteristic):
+    def add_characteristic(self, characteristic: BleakGATTCharacteristic) -> None:
         """Add a :py:class:`~BleakGATTCharacteristic` to the service.
 
         Should not be used by end user, but rather by `bleak` itself.
@@ -81,7 +81,7 @@ def get_characteristic(
 class BleakGATTServiceCollection:
     """Simple data container for storing the peripheral's service complement."""
 
-    def __init__(self):
+    def __init__(self) -> None:
         self.__services = {}
         self.__characteristics = {}
         self.__descriptors = {}
@@ -117,7 +117,7 @@ def descriptors(self) -> Dict[int, BleakGATTDescriptor]:
         """Returns a dictionary of integer handles mapping to BleakGATTDescriptor"""
         return self.__descriptors
 
-    def add_service(self, service: BleakGATTService):
+    def add_service(self, service: BleakGATTService) -> None:
         """Add a :py:class:`~BleakGATTService` to the service collection.
 
         Should not be used by end user, but rather by `bleak` itself.
@@ -153,7 +153,7 @@ def get_service(
 
         return x[0] if x else None
 
-    def add_characteristic(self, characteristic: BleakGATTCharacteristic):
+    def add_characteristic(self, characteristic: BleakGATTCharacteristic) -> None:
         """Add a :py:class:`~BleakGATTCharacteristic` to the service collection.
 
         Should not be used by end user, but rather by `bleak` itself.
@@ -191,7 +191,7 @@ def get_characteristic(
 
         return x[0] if x else None
 
-    def add_descriptor(self, descriptor: BleakGATTDescriptor):
+    def add_descriptor(self, descriptor: BleakGATTDescriptor) -> None:
         """Add a :py:class:`~BleakGATTDescriptor` to the service collection.
 
         Should not be used by end user, but rather by `bleak` itself.
diff --git a/bleak/uuids.py b/bleak/uuids.py
index 0b6a17d6..61338158 100644
--- a/bleak/uuids.py
+++ b/bleak/uuids.py
@@ -1177,7 +1177,7 @@
 }
 
 
-def uuidstr_to_str(uuid_):
+def uuidstr_to_str(uuid_: str) -> str:
     uuid_ = uuid_.lower()
     s = uuid128_dict.get(uuid_)
     if s: