diff --git a/google/cloud/pubsublite/cloudpubsub/internal/assigning_subscriber.py b/google/cloud/pubsublite/cloudpubsub/internal/assigning_subscriber.py
index c2706113..0ecde27d 100644
--- a/google/cloud/pubsublite/cloudpubsub/internal/assigning_subscriber.py
+++ b/google/cloud/pubsublite/cloudpubsub/internal/assigning_subscriber.py
@@ -17,6 +17,7 @@
 
 from google.cloud.pubsub_v1.subscriber.message import Message
 
+from google.cloud.pubsublite.cloudpubsub.reassignment_handler import ReassignmentHandler
 from google.cloud.pubsublite.cloudpubsub.internal.single_subscriber import (
     AsyncSingleSubscriber,
 )
@@ -39,6 +40,7 @@ class _RunningSubscriber(NamedTuple):
 class AssigningSingleSubscriber(AsyncSingleSubscriber, PermanentFailable):
     _assigner_factory: Callable[[], Assigner]
     _subscriber_factory: PartitionSubscriberFactory
+    _reassignment_handler: ReassignmentHandler
 
     _subscribers: Dict[Partition, _RunningSubscriber]
 
@@ -51,6 +53,7 @@ def __init__(
         self,
         assigner_factory: Callable[[], Assigner],
         subscriber_factory: PartitionSubscriberFactory,
+        reassignment_handler: ReassignmentHandler,
     ):
         """
         Accepts a factory for an Assigner instead of an Assigner because GRPC asyncio uses the current thread's event
@@ -58,8 +61,9 @@ def __init__(
         """
         super().__init__()
         self._assigner_factory = assigner_factory
-        self._assigner = None
         self._subscriber_factory = subscriber_factory
+        self._reassignment_handler = reassignment_handler
+        self._assigner = None
         self._subscribers = {}
         self._batches = None
 
@@ -85,14 +89,20 @@ async def _stop_subscriber(self, running: _RunningSubscriber):
 
     async def _assign_action(self):
         assignment: Set[Partition] = await self._assigner.get_assignment()
-        added_partitions = assignment - self._subscribers.keys()
-        removed_partitions = self._subscribers.keys() - assignment
+        old_assignment: Set[Partition] = set(self._subscribers.keys())
+        added_partitions = assignment - old_assignment
+        removed_partitions = old_assignment - assignment
         for partition in added_partitions:
             await self._start_subscriber(partition)
         for partition in removed_partitions:
             subscriber = self._subscribers[partition]
             del self._subscribers[partition]
             await self._stop_subscriber(subscriber)
+        maybe_awaitable = self._reassignment_handler.handle_reassignment(
+            old_assignment, assignment
+        )
+        if maybe_awaitable is not None:
+            await maybe_awaitable
 
     async def __aenter__(self):
         self._batches = Queue()
diff --git a/google/cloud/pubsublite/cloudpubsub/internal/make_subscriber.py b/google/cloud/pubsublite/cloudpubsub/internal/make_subscriber.py
index e1ffff55..58e12353 100644
--- a/google/cloud/pubsublite/cloudpubsub/internal/make_subscriber.py
+++ b/google/cloud/pubsublite/cloudpubsub/internal/make_subscriber.py
@@ -18,6 +18,10 @@
 from google.api_core.client_options import ClientOptions
 from google.auth.credentials import Credentials
 
+from google.cloud.pubsublite.cloudpubsub.reassignment_handler import (
+    ReassignmentHandler,
+    DefaultReassignmentHandler,
+)
 from google.cloud.pubsublite.cloudpubsub.message_transforms import (
     to_cps_subscribe_message,
     add_id_to_cps_subscribe_transformer,
@@ -179,6 +183,7 @@ def make_async_subscriber(
     transport: str,
     per_partition_flow_control_settings: FlowControlSettings,
     nack_handler: Optional[NackHandler] = None,
+    reassignment_handler: Optional[ReassignmentHandler] = None,
     message_transformer: Optional[MessageTransformer] = None,
     fixed_partitions: Optional[Set[Partition]] = None,
     credentials: Optional[Credentials] = None,
@@ -218,6 +223,8 @@ def make_async_subscriber(
 
     if nack_handler is None:
         nack_handler = DefaultNackHandler()
+    if reassignment_handler is None:
+        reassignment_handler = DefaultReassignmentHandler()
     if message_transformer is None:
         message_transformer = MessageTransformer.of_callable(to_cps_subscribe_message)
     partition_subscriber_factory = _make_partition_subscriber_factory(
@@ -230,4 +237,6 @@ def make_async_subscriber(
         nack_handler,
         message_transformer,
     )
-    return AssigningSingleSubscriber(assigner_factory, partition_subscriber_factory)
+    return AssigningSingleSubscriber(
+        assigner_factory, partition_subscriber_factory, reassignment_handler
+    )
diff --git a/google/cloud/pubsublite/cloudpubsub/nack_handler.py b/google/cloud/pubsublite/cloudpubsub/nack_handler.py
index a79dca10..b55501d4 100644
--- a/google/cloud/pubsublite/cloudpubsub/nack_handler.py
+++ b/google/cloud/pubsublite/cloudpubsub/nack_handler.py
@@ -28,6 +28,8 @@ class NackHandler(ABC):
     def on_nack(self, message: PubsubMessage, ack: Callable[[], None]):
         """Handle a negative acknowledgement. ack must eventually be called.
 
+        This method will be called on an event loop and should not block.
+
         Args:
           message: The nacked message.
           ack: A callable to acknowledge the underlying message. This must eventually be called.
diff --git a/google/cloud/pubsublite/cloudpubsub/publisher_client.py b/google/cloud/pubsublite/cloudpubsub/publisher_client.py
index 9ddd4da5..248465bd 100644
--- a/google/cloud/pubsublite/cloudpubsub/publisher_client.py
+++ b/google/cloud/pubsublite/cloudpubsub/publisher_client.py
@@ -62,6 +62,7 @@ class PublisherClient(PublisherClientInterface, ConstructableFromServiceAccount)
 
     def __init__(
         self,
+        *,
         per_partition_batching_settings: Optional[BatchSettings] = None,
         credentials: Optional[Credentials] = None,
         transport: str = "grpc_asyncio",
@@ -93,7 +94,7 @@ def publish(
         topic: Union[TopicPath, str],
         data: bytes,
         ordering_key: str = "",
-        **attrs: Mapping[str, str]
+        **attrs: Mapping[str, str],
     ) -> "Future[str]":
         self._require_stared.require_started()
         return self._impl.publish(
@@ -132,6 +133,7 @@ class AsyncPublisherClient(
 
     def __init__(
         self,
+        *,
         per_partition_batching_settings: Optional[BatchSettings] = None,
         credentials: Optional[Credentials] = None,
         transport: str = "grpc_asyncio",
@@ -163,7 +165,7 @@ async def publish(
         topic: Union[TopicPath, str],
         data: bytes,
         ordering_key: str = "",
-        **attrs: Mapping[str, str]
+        **attrs: Mapping[str, str],
     ) -> str:
         self._require_stared.require_started()
         return await self._impl.publish(
diff --git a/google/cloud/pubsublite/cloudpubsub/reassignment_handler.py b/google/cloud/pubsublite/cloudpubsub/reassignment_handler.py
new file mode 100644
index 00000000..5cc6a668
--- /dev/null
+++ b/google/cloud/pubsublite/cloudpubsub/reassignment_handler.py
@@ -0,0 +1,70 @@
+# Copyright 2020 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from abc import ABC, abstractmethod
+from typing import Set, Optional, Awaitable
+
+from google.cloud.pubsublite.types import Partition
+
+
+class ReassignmentHandler(ABC):
+    """
+    A ReassignmentHandler is called any time a new partition assignment is received from the server.
+    It will be called with both the previous and new assignments as decided by the backend.
+
+    The client library will not acknowledge the assignment until handleReassignment returns. The
+    assigning backend will not assign any of the partitions in `before` to another server unless the
+    assignment is acknowledged, or a client takes too long to acknowledged (currently 30 seconds from
+    the time the assignment is sent from server's point of view).
+
+    Because of the above, as long as reassignment handling is processed quickly, it can be used to
+    abort outstanding operations on partitions which are being assigned away from this client, or to
+    pre-warm state which will be used by the MessageCallback.
+    """
+
+    @abstractmethod
+    def handle_reassignment(
+        self, before: Set[Partition], after: Set[Partition]
+    ) -> Optional[Awaitable]:
+        """
+        Called with the previous and new assignment delivered to this client on an assignment change.
+        The assignment will not be acknowledged until this method returns, so it should complete
+        quickly, or the backend will assume it is non-responsive and assign all partitions away without
+        waiting for acknowledgement.
+
+        handle_reassignment will only be called after no new message deliveries will be started for the partition.
+        There may still be messages in flight on executors or in async callbacks.
+
+        Acks or nacks on messages from partitions being assigned away will have no effect.
+
+        This method will be called on an event loop and should not block.
+
+        Args:
+          before: The previous assignment.
+          after: The new assignment.
+
+        Returns:
+          Either None or an Awaitable to be waited on before acknowledging reassignment.
+
+        Raises:
+          GoogleAPICallError: To fail the client if raised.
+        """
+        pass
+
+
+class DefaultReassignmentHandler(ReassignmentHandler):
+    def handle_reassignment(
+        self, before: Set[Partition], after: Set[Partition]
+    ) -> Optional[Awaitable]:
+        return None
diff --git a/google/cloud/pubsublite/cloudpubsub/subscriber_client.py b/google/cloud/pubsublite/cloudpubsub/subscriber_client.py
index eda098fb..ab2b98cd 100644
--- a/google/cloud/pubsublite/cloudpubsub/subscriber_client.py
+++ b/google/cloud/pubsublite/cloudpubsub/subscriber_client.py
@@ -20,6 +20,7 @@
 from google.cloud.pubsub_v1.subscriber.futures import StreamingPullFuture
 from google.cloud.pubsub_v1.subscriber.message import Message
 
+from google.cloud.pubsublite.cloudpubsub.reassignment_handler import ReassignmentHandler
 from google.cloud.pubsublite.cloudpubsub.internal.make_subscriber import (
     make_async_subscriber,
 )
@@ -61,8 +62,10 @@ class SubscriberClient(SubscriberClientInterface, ConstructableFromServiceAccoun
 
     def __init__(
         self,
+        *,
         executor: Optional[ThreadPoolExecutor] = None,
         nack_handler: Optional[NackHandler] = None,
+        reassignment_handler: Optional[ReassignmentHandler] = None,
         message_transformer: Optional[MessageTransformer] = None,
         credentials: Optional[Credentials] = None,
         transport: str = "grpc_asyncio",
@@ -88,6 +91,7 @@ def __init__(
                 transport=transport,
                 per_partition_flow_control_settings=settings,
                 nack_handler=nack_handler,
+                reassignment_handler=reassignment_handler,
                 message_transformer=message_transformer,
                 fixed_partitions=partitions,
                 credentials=credentials,
@@ -140,7 +144,9 @@ class AsyncSubscriberClient(
 
     def __init__(
         self,
+        *,
         nack_handler: Optional[NackHandler] = None,
+        reassignment_handler: Optional[ReassignmentHandler] = None,
         message_transformer: Optional[MessageTransformer] = None,
         credentials: Optional[Credentials] = None,
         transport: str = "grpc_asyncio",
@@ -162,6 +168,7 @@ def __init__(
                 transport=transport,
                 per_partition_flow_control_settings=settings,
                 nack_handler=nack_handler,
+                reassignment_handler=reassignment_handler,
                 message_transformer=message_transformer,
                 fixed_partitions=partitions,
                 credentials=credentials,
diff --git a/google/cloud/pubsublite/internal/wire/permanent_failable.py b/google/cloud/pubsublite/internal/wire/permanent_failable.py
index d96c4c9d..ae78c861 100644
--- a/google/cloud/pubsublite/internal/wire/permanent_failable.py
+++ b/google/cloud/pubsublite/internal/wire/permanent_failable.py
@@ -85,8 +85,10 @@ async def run_poller(self, poll_action: Callable[[], Awaitable[None]]):
         try:
             while True:
                 await self.await_unless_failed(poll_action())
-        except GoogleAPICallError as e:
-            self.fail(e)
+        except asyncio.CancelledError:
+            pass
+        except Exception as e:
+            self.fail(adapt_error(e))
 
     def fail(self, err: GoogleAPICallError):
         if not self._failure_task.done():
diff --git a/tests/unit/pubsublite/cloudpubsub/internal/assigning_subscriber_test.py b/tests/unit/pubsublite/cloudpubsub/internal/assigning_subscriber_test.py
index 50a39ae0..2232d4c0 100644
--- a/tests/unit/pubsublite/cloudpubsub/internal/assigning_subscriber_test.py
+++ b/tests/unit/pubsublite/cloudpubsub/internal/assigning_subscriber_test.py
@@ -14,6 +14,7 @@
 
 from typing import Set
 
+from asyncio import Future
 from asynctest.mock import MagicMock, call
 import threading
 import pytest
@@ -21,6 +22,7 @@
 from google.cloud.pubsub_v1.subscriber.message import Message
 from google.pubsub_v1 import PubsubMessage
 
+from google.cloud.pubsublite.cloudpubsub.reassignment_handler import ReassignmentHandler
 from google.cloud.pubsublite.cloudpubsub.internal.assigning_subscriber import (
     AssigningSingleSubscriber,
     PartitionSubscriberFactory,
@@ -46,17 +48,26 @@ def assigner():
     return mock_async_context_manager(MagicMock(spec=Assigner))
 
 
+@pytest.fixture()
+def reassignment_handler():
+    handler = MagicMock(spec=ReassignmentHandler)
+    handler.handle_reassignment.return_value = None
+    return handler
+
+
 @pytest.fixture()
 def subscriber_factory():
     return MagicMock(spec=PartitionSubscriberFactory)
 
 
 @pytest.fixture()
-def subscriber(assigner, subscriber_factory):
+def subscriber(assigner, subscriber_factory, reassignment_handler):
     box = Box()
 
     def set_box():
-        box.val = AssigningSingleSubscriber(lambda: assigner, subscriber_factory)
+        box.val = AssigningSingleSubscriber(
+            lambda: assigner, subscriber_factory, reassignment_handler
+        )
 
     # Initialize AssigningSubscriber on another thread with a different event loop.
     thread = threading.Thread(target=set_box)
@@ -103,7 +114,9 @@ async def test_assigner_failure(subscriber, assigner, subscriber_factory):
             await subscriber.read()
 
 
-async def test_assignment_change(subscriber, assigner, subscriber_factory):
+async def test_assignment_change(
+    subscriber, assigner, subscriber_factory, reassignment_handler
+):
     assign_queues = wire_queues(assigner.get_assignment)
     async with subscriber:
         await assign_queues.called.get()
@@ -124,13 +137,35 @@ async def test_assignment_change(subscriber, assigner, subscriber_factory):
         )
         sub1.__aenter__.assert_called_once()
         sub2.__aenter__.assert_called_once()
+        reassignment_handler.handle_reassignment.assert_called_once_with(
+            set(), {Partition(1), Partition(2)}
+        )
+        reassignment_triggered = Future()
+        reassignment_future = Future()
+
+        def on_reassignment(before, after):
+            reassignment_triggered.set_result(None)
+            return reassignment_future
+
+        reassignment_handler.handle_reassignment.side_effect = on_reassignment
         await assign_queues.results.put({Partition(1), Partition(3)})
-        await assign_queues.called.get()
+        await reassignment_triggered
         subscriber_factory.assert_has_calls(
             [call(Partition(1)), call(Partition(2)), call(Partition(3))], any_order=True
         )
         sub3.__aenter__.assert_called_once()
         sub2.__aexit__.assert_called_once()
+        reassignment_handler.handle_reassignment.assert_has_calls(
+            [
+                call(set(), {Partition(1), Partition(2)}),
+                call({Partition(1), Partition(2)}, {Partition(1), Partition(3)}),
+            ]
+        )
+        # Assigner is stalled waiting for reassignment future completion
+        assert assign_queues.called.empty()
+        reassignment_future.set_result(None)
+        await assign_queues.called.get()
+
     sub1.__aexit__.assert_called_once()
     sub2.__aexit__.assert_called_once()
     sub3.__aexit__.assert_called_once()
diff --git a/tests/unit/pubsublite/cloudpubsub/internal/single_partition_subscriber_test.py b/tests/unit/pubsublite/cloudpubsub/internal/single_partition_subscriber_test.py
index 3d355642..c007913a 100644
--- a/tests/unit/pubsublite/cloudpubsub/internal/single_partition_subscriber_test.py
+++ b/tests/unit/pubsublite/cloudpubsub/internal/single_partition_subscriber_test.py
@@ -226,6 +226,27 @@ async def sleep_forever():
             await subscriber.read()
 
 
+async def test_nack_after_shutdown_noop(
+    subscriber: SinglePartitionSingleSubscriber,
+    underlying,
+    transformer,
+    ack_set_tracker,
+    nack_handler,
+):
+    async with subscriber:
+        message = SequencedMessage(cursor=Cursor(offset=1), size_bytes=5)._pb
+        underlying.read.return_value = [message]
+        read: List[Message] = await subscriber.read()
+        assert len(read) == 1
+        ack_set_tracker.track.assert_has_calls([call(1)])
+        nack_handler.on_nack.side_effect = FailedPrecondition("Bad nack")
+    read[0].nack()
+    # Yield to the runtime, allowing the subscriber to process the queue if it
+    # is still running (which it should not be)
+    await asyncio.sleep(1)
+    nack_handler.on_nack.assert_has_calls([])
+
+
 async def test_nack_calls_ack(
     subscriber: SinglePartitionSingleSubscriber,
     underlying,