diff --git a/CHANGELOG.md b/CHANGELOG.md
index c455722..612e496 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- OnStateChanged method
+
 ## [0.1.0] - 2025-04-23
 
 ### Added
diff --git a/TJC.StateMachine.Tests/Mocks/RevolverMock.cs b/TJC.StateMachine.Tests/Mocks/RevolverMock.cs
index dcc6256..af389b0 100644
--- a/TJC.StateMachine.Tests/Mocks/RevolverMock.cs
+++ b/TJC.StateMachine.Tests/Mocks/RevolverMock.cs
@@ -1,10 +1,11 @@
 namespace TJC.StateMachine.Tests.Mocks
 {
-    internal class RevolverMock()
-        : StateMachineBase<RevolverStates>(RevolverStates.Loaded)
+    internal class RevolverMock() : StateMachineBase<RevolverStates>(RevolverStates.Loaded)
     {
         public int BulletsLoaded { get; private set; } = 6;
 
+        internal uint StateChanges = 0;
+
         public bool TryShoot()
         {
             switch (State)
@@ -17,7 +18,9 @@ public bool TryShoot()
                         State = RevolverStates.Empty;
                     return true;
                 default:
-                    throw new InvalidOperationException($"Unknown State [{State}] for method {nameof(TryShoot)}");
+                    throw new InvalidOperationException(
+                        $"Unknown State [{State}] for method {nameof(TryShoot)}"
+                    );
             }
         }
 
@@ -26,5 +29,10 @@ public void Reload()
             BulletsLoaded = 6;
             State = RevolverStates.Loaded;
         }
+
+        protected override void OnStateChanged()
+        {
+            StateChanges++;
+        }
     }
-}
\ No newline at end of file
+}
diff --git a/TJC.StateMachine.Tests/Tests/RevolverMockTests.cs b/TJC.StateMachine.Tests/Tests/RevolverMockTests.cs
index 6156f15..9289925 100644
--- a/TJC.StateMachine.Tests/Tests/RevolverMockTests.cs
+++ b/TJC.StateMachine.Tests/Tests/RevolverMockTests.cs
@@ -57,5 +57,40 @@ public void EnsureReloadingResetsBulletsTo6()
             revolver.Reload();
             Assert.AreEqual(6, revolver.BulletsLoaded);
         }
+
+        [TestMethod]
+        public void EnsureEmptyingChangesStateAndReloadingChangesStateAgain()
+        {
+            var revolver = new RevolverMock();
+
+            Assert.AreEqual<uint>(0, revolver.StateChanges);
+
+            revolver.TryShoot();
+            revolver.TryShoot();
+            revolver.TryShoot();
+            revolver.TryShoot();
+            revolver.TryShoot();
+            revolver.TryShoot();
+
+            Assert.AreEqual<uint>(1, revolver.StateChanges);
+
+            revolver.Reload();
+
+            Assert.AreEqual<uint>(2, revolver.StateChanges);
+        }
+
+        [TestMethod]
+        public void EnsureReloadingMultipleTimesDoesNotChangeState()
+        {
+            var revolver = new RevolverMock();
+
+            Assert.AreEqual<uint>(0, revolver.StateChanges);
+
+            revolver.Reload();
+            revolver.Reload();
+            revolver.Reload();
+
+            Assert.AreEqual<uint>(0, revolver.StateChanges);
+        }
     }
 }
diff --git a/TJC.StateMachine/StateMachineBase.cs b/TJC.StateMachine/StateMachineBase.cs
index 6e0fc99..8c33b4f 100644
--- a/TJC.StateMachine/StateMachineBase.cs
+++ b/TJC.StateMachine/StateMachineBase.cs
@@ -5,11 +5,30 @@
     /// </summary>
     /// <typeparam name="T">State Type.</typeparam>
     /// <param name="initialState">Initial State of the State Machine.</param>
-    public class StateMachineBase<T>(T initialState)
+    public abstract class StateMachineBase<T>(T initialState)
     {
+        private T _state = initialState;
+
         /// <summary>
         /// State of the state machine.
         /// </summary>
-        protected T State { get; set; } = initialState;
+        protected T State
+        {
+            get => _state;
+            set
+            {
+                if (Equals(_state, value))
+                    return;
+                _state = value;
+                OnStateChanged();
+            }
+        }
+
+        /// <summary>
+        /// Called on state change.
+        /// Can be used to add tracing &amp; debugging.
+        /// Should not be used to notify of state changes externally.
+        /// </summary>
+        protected abstract void OnStateChanged();
     }
-}
\ No newline at end of file
+}