diff --git a/src/AWS.Messaging/Services/DefaultMessageManager.cs b/src/AWS.Messaging/Services/DefaultMessageManager.cs
index 8fb6948..1788bd3 100644
--- a/src/AWS.Messaging/Services/DefaultMessageManager.cs
+++ b/src/AWS.Messaging/Services/DefaultMessageManager.cs
@@ -36,7 +36,8 @@ public DefaultMessageManager(IMessagePoller messagePoller, IHandlerInvoker handl
     }
 
     /// <inheritdoc/>
-    public int ActiveMessageCount {
+    public int ActiveMessageCount
+    {
         get
         {
             lock (_activeMessageCountLock)
@@ -44,22 +45,30 @@ public int ActiveMessageCount {
                 return _activeMessageCount;
             }
         }
-        set
+    }
+
+    /// <summary>
+    /// Updates <see cref="ActiveMessageCount"/> in a threadsafe manner
+    /// </summary>
+    /// <param name="delta">Difference to apply to the current active message count</param>
+    /// <returns>Updated <see cref="ActiveMessageCount"/></returns>
+    private int UpdateActiveMessageCount(int delta)
+    {
+        lock (_activeMessageCountLock)
         {
-            lock (_activeMessageCountLock)
-            {
-                _logger.LogTrace("Updating {activeMessageCount} from {before} to {after}", nameof(ActiveMessageCount), ActiveMessageCount, value);
+            var newValue = _activeMessageCount + delta;
+            _logger.LogTrace("Updating {activeMessageCount} from {before} to {after}", nameof(ActiveMessageCount), _activeMessageCount, newValue);
 
-                var isDecrementing = value < _activeMessageCount;
-                _activeMessageCount = value;
+            _activeMessageCount = newValue;
 
-                // If we just decremented the active message count, signal to the poller
-                // that there may be more capacity available again.
-                if (isDecrementing)
-                {
-                    _activeMessageCountDecrementedEvent.Set();
-                }
+            // If we just decremented the active message count, signal to the poller
+            // that there may be more capacity available again.
+            if (delta < 0)
+            {
+                _activeMessageCountDecrementedEvent.Set();
             }
+
+            return _activeMessageCount;
         }
     }
 
@@ -86,11 +95,10 @@ public Task WaitAsync(TimeSpan timeout)
         return Task.CompletedTask;
     }
 
-
     /// <inheritdoc/>
     public async Task ProcessMessageAsync(MessageEnvelope messageEnvelope, SubscriberMapping subscriberMapping, CancellationToken token = default)
     {
-        ActiveMessageCount++;
+        UpdateActiveMessageCount(1);
 
         var handlerTask = _handlerInvoker.InvokeAsync(messageEnvelope, subscriberMapping, token);
 
@@ -132,7 +140,7 @@ public async Task ProcessMessageAsync(MessageEnvelope messageEnvelope, Subscribe
             _logger.LogError(handlerTask.Exception, "Message handling failed unexpectedly for message ID {messageId}", messageEnvelope.Id);
         }
 
-        ActiveMessageCount--;
+        UpdateActiveMessageCount(-1);
     }
 
     /// <summary>
@@ -149,6 +157,8 @@ private void StartMessageVisibilityExtensionTaskIfNotRunning(CancellationToken t
                 if (_visibilityTimeoutExtensionTask == null || _visibilityTimeoutExtensionTask.IsCompleted)
                 {
                     _visibilityTimeoutExtensionTask = ExtendUnfinishedMessageVisibilityTimeoutBatch(token);
+
+                    _logger.LogTrace("Started task with id {id} to extend the visibility timeout of in flight messages", _visibilityTimeoutExtensionTask.Id);
                 }
             }
         }
diff --git a/test/AWS.Messaging.UnitTests/DefaultMessageManagerTests.cs b/test/AWS.Messaging.UnitTests/DefaultMessageManagerTests.cs
index 6612929..a9de73b 100644
--- a/test/AWS.Messaging.UnitTests/DefaultMessageManagerTests.cs
+++ b/test/AWS.Messaging.UnitTests/DefaultMessageManagerTests.cs
@@ -57,6 +57,9 @@ public async Task DefaultMessageManager_ManagesMessageSuccess()
             Assert.Equal(0, manager.ActiveMessageCount);
         }
 
+        /// <summary>
+        /// Happy path test for a failed message handling, that it is not deleted from the queue at the end
+        /// </summary>
         [Fact]
         public async Task DefaultMessageManager_ManagesMessageFailed()
         {
@@ -93,6 +96,10 @@ public async Task DefaultMessageManager_ManagesMessageFailed()
             Assert.Equal(0, manager.ActiveMessageCount);
         }
 
+        /// <summary>
+        /// Tests that the manager extends the visibility timeout when the message handler
+        /// takes longer than the refresh interval
+        /// </summary>
         [Fact]
         public async Task DefaultMessageManager_RefreshesLongHandler()
         {
@@ -135,6 +142,36 @@ public async Task DefaultMessageManager_RefreshesLongHandler()
             Assert.Equal(0, manager.ActiveMessageCount);
         }
 
+        /// <summary>
+        /// Queues many message handling tasks for a single message manager to ensure that it
+        /// is managing its active message count in a threadsafe manner
+        /// </summary>
+        [Fact]
+        public async Task DefaultMessageManager_CountsActiveMessagesCorrectly()
+        {
+            var mockPoller = CreateMockPoller(messageVisibilityRefreshInterval: 1);
+            var mockHandlerInvoker = CreateMockHandlerInvoker(MessageProcessStatus.Success(), TimeSpan.FromSeconds(1));
+
+            var manager = new DefaultMessageManager(mockPoller.Object, mockHandlerInvoker.Object, new NullLogger<DefaultMessageManager>());
+            var subscriberMapping = new SubscriberMapping(typeof(ChatMessageHandler), typeof(ChatMessage));
+           
+            var tasks = new List<Task>();
+
+            for (int i = 0; i < 100; i++)
+            {
+                var messsageEnvelope = new MessageEnvelope<ChatMessage>()
+                {
+                    Id = i.ToString()
+                };
+                tasks.Add(manager.ProcessMessageAsync(messsageEnvelope, subscriberMapping));
+            }
+
+            await Task.WhenAll(tasks);
+
+            // Verify that the active message count was deprecated back to 0
+            Assert.Equal(0, manager.ActiveMessageCount);
+        }
+
         /// <summary>
         /// Mocks a message poller with the given visibility refresh interval
         /// </summary>