diff --git a/src/Stunts.UnitTests/Scenarios/Synchronized.cs b/src/Stunts.UnitTests/Scenarios/Synchronized.cs
new file mode 100644
index 0000000..fa11946
--- /dev/null
+++ b/src/Stunts.UnitTests/Scenarios/Synchronized.cs
@@ -0,0 +1,209 @@
+#pragma warning disable IDE0079
+#pragma warning disable CS0436
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Stunts;
+using Xunit;
+
+namespace Stunts.Scenarios.Synchronized
+{
+    public interface IWorker
+    {
+        int Work(int id);
+        Task<int> WorkAsync(int id);
+        ValueTask<int> WorkValueAsync(int id);
+        int Reentrant();
+    }
+
+    /// <summary>
+    /// A synchronized behavior serializes overlapping calls using a standard lock.
+    /// </summary>
+    public class Test : IRunnable
+    {
+        public void Run()
+        {
+            OverlappingSyncCalls_RunOneAtATime();
+            ReentrantCall_Succeeds_WithReentrantLock();
+            SharedLock_CoordinatesTwoStunts();
+            DefaultLock_DoesNotCoordinateTwoStunts();
+        }
+
+        public void OverlappingSyncCalls_RunOneAtATime()
+        {
+            var stunt = Stunt.For<IWorker>();
+            stunt.AddBehavior(new SynchronizedBehavior());
+            var work = new TrackingWork();
+            stunt.AddBehavior(work);
+            IWorker worker = stunt.ToObject();
+
+            var barrier = new Barrier(2);
+            var tasks = new Task[2];
+            for (int i = 0; i < 2; i++)
+                tasks[i] = Task.Run(() =>
+                {
+                    barrier.SignalAndWait();
+                    worker.Work(1);
+                });
+
+            Task.WaitAll(tasks);
+
+            Assert.Equal(1, work.MaxConcurrent);
+            Assert.Equal(2, work.Calls);
+        }
+
+        public void ReentrantCall_Succeeds_WithReentrantLock()
+        {
+            var stunt = Stunt.For<IWorker>();
+            stunt.AddBehavior(new SynchronizedBehavior());
+            IWorker? worker = null;
+            stunt.AddBehavior(new ReentrantWork(v => worker!.Work(1)));
+            stunt.AddBehavior(new LambdaWork(invocation =>
+                invocation.MethodBase.Name == nameof(IWorker.Work)
+                    ? new MethodReturn(invocation, 0, invocation.Arguments)
+                    : new MethodReturn(invocation, 0, invocation.Arguments)));
+            worker = stunt.ToObject();
+
+            // Reentrant Monitor: same-thread re-entry succeeds (no deadlock, no throw).
+            var result = worker.Reentrant();
+            Assert.Equal(0, result);
+        }
+
+        public void SharedLock_CoordinatesTwoStunts()
+        {
+            var syncRoot = new object();
+            var barrier = new Barrier(2);
+            int concurrent = 0;
+            int maxConcurrent = 0;
+
+            IWorker Make()
+            {
+                var stunt = Stunt.For<IWorker>();
+                stunt.AddBehavior(new SynchronizedBehavior(syncRoot));
+                stunt.AddBehavior(new LambdaWork(invocation =>
+                {
+                    var c = Interlocked.Increment(ref concurrent);
+                    int max;
+                    do { max = maxConcurrent; }
+                    while (c > max && Interlocked.CompareExchange(ref maxConcurrent, c, max) != max);
+                    try
+                    {
+                        barrier.SignalAndWait(5000);
+                        Thread.Sleep(50);
+                        return new MethodReturn(invocation, 0, invocation.Arguments);
+                    }
+                    finally
+                    {
+                        Interlocked.Decrement(ref concurrent);
+                    }
+                }));
+                return stunt.ToObject();
+            }
+
+            var w1 = Make();
+            var w2 = Make();
+
+            var t1 = Task.Run(() => w1.Work(1));
+            var t2 = Task.Run(() => w2.Work(2));
+            Task.WaitAll(t1, t2);
+
+            Assert.Equal(1, maxConcurrent);
+        }
+
+        public void DefaultLock_DoesNotCoordinateTwoStunts()
+        {
+            var barrier = new Barrier(2);
+            int concurrent = 0;
+            int maxConcurrent = 0;
+
+            IWorker Make()
+            {
+                var stunt = Stunt.For<IWorker>();
+                stunt.AddBehavior(new SynchronizedBehavior());
+                stunt.AddBehavior(new LambdaWork(invocation =>
+                {
+                    var c = Interlocked.Increment(ref concurrent);
+                    int max;
+                    do { max = maxConcurrent; }
+                    while (c > max && Interlocked.CompareExchange(ref maxConcurrent, c, max) != max);
+                    try
+                    {
+                        barrier.SignalAndWait(5000);
+                        Thread.Sleep(50);
+                        return new MethodReturn(invocation, 0, invocation.Arguments);
+                    }
+                    finally
+                    {
+                        Interlocked.Decrement(ref concurrent);
+                    }
+                }));
+                return stunt.ToObject();
+            }
+
+            var w1 = Make();
+            var w2 = Make();
+
+            var t1 = Task.Run(() => w1.Work(1));
+            var t2 = Task.Run(() => w2.Work(2));
+            Task.WaitAll(t1, t2);
+
+            Assert.Equal(2, maxConcurrent);
+        }
+
+        class TrackingWork : IStuntBehavior
+        {
+            public int Calls;
+            public int MaxConcurrent;
+            int current;
+
+            public bool AppliesTo(IMethodInvocation invocation)
+                => invocation.MethodBase.Name == nameof(IWorker.Work);
+
+            public IMethodReturn Execute(IMethodInvocation invocation, ExecuteHandler next)
+            {
+                var c = Interlocked.Increment(ref current);
+                int max;
+                do { max = MaxConcurrent; }
+                while (c > max && Interlocked.CompareExchange(ref MaxConcurrent, c, max) != max);
+                try
+                {
+                    Interlocked.Increment(ref Calls);
+                    Thread.Sleep(50);
+                    return new MethodReturn(invocation, 0, invocation.Arguments);
+                }
+                finally
+                {
+                    Interlocked.Decrement(ref current);
+                }
+            }
+        }
+
+        class ReentrantWork : IStuntBehavior
+        {
+            readonly Action<IMethodInvocation> reenter;
+
+            public ReentrantWork(Action<IMethodInvocation> reenter) => this.reenter = reenter;
+
+            public bool AppliesTo(IMethodInvocation invocation)
+                => invocation.MethodBase.Name == nameof(IWorker.Reentrant);
+
+            public IMethodReturn Execute(IMethodInvocation invocation, ExecuteHandler next)
+            {
+                reenter(invocation);
+                return new MethodReturn(invocation, 0, invocation.Arguments);
+            }
+        }
+
+        class LambdaWork : IStuntBehavior
+        {
+            readonly Func<IMethodInvocation, IMethodReturn> handler;
+
+            public LambdaWork(Func<IMethodInvocation, IMethodReturn> handler) => this.handler = handler;
+
+            public bool AppliesTo(IMethodInvocation invocation) => true;
+
+            public IMethodReturn Execute(IMethodInvocation invocation, ExecuteHandler next)
+                => handler(invocation);
+        }
+    }
+}
diff --git a/src/Stunts/SynchronizedBehavior.cs b/src/Stunts/SynchronizedBehavior.cs
new file mode 100644
index 0000000..a0b9dc9
--- /dev/null
+++ b/src/Stunts/SynchronizedBehavior.cs
@@ -0,0 +1,76 @@
+using System;
+
+namespace Stunts
+{
+    /// <summary>
+    /// An <see cref="IStuntBehavior"/> that serializes overlapping calls on a stunt
+    /// using a standard <see cref="object"/> lock.
+    /// </summary>
+    /// <remarks>
+    /// The lock is acquired before <c>next</c> runs and released when it returns,
+    /// following the normal <c>lock</c> statement semantics (reentrant).
+    /// <para>
+    /// By default each behavior owns a private lock object, so one behavior locks
+    /// one stunt. Pass an explicit <c>syncRoot</c> to coordinate multiple behaviors
+    /// on the same lock.
+    /// </para>
+    /// <para>
+    /// The behavior implements <see cref="ICloneable"/>: the default private lock
+    /// is fresh per clone, while an explicit lock object stays shared across clones.
+    /// </para>
+    /// </remarks>
+    public class SynchronizedBehavior : IStuntBehavior, ICloneable
+    {
+        readonly object syncRoot;
+        readonly bool isShared;
+
+        /// <summary>
+        /// Initializes the behavior with a private lock owned by the behavior.
+        /// </summary>
+        public SynchronizedBehavior() : this(new object(), false) { }
+
+        /// <summary>
+        /// Initializes the behavior.
+        /// </summary>
+        /// <param name="syncRoot">
+        /// Optional lock object. When <see langword="null"/> (default), the behavior
+        /// uses a private object, so one behavior locks one stunt. When provided,
+        /// the behavior locks on that object.
+        /// </param>
+        public SynchronizedBehavior(object? syncRoot)
+            : this(syncRoot ?? new object(), syncRoot != null)
+        {
+        }
+
+        SynchronizedBehavior(object syncRoot, bool isShared)
+        {
+            this.syncRoot = syncRoot;
+            this.isShared = isShared;
+        }
+
+        /// <summary>
+        /// Returns a new <see cref="SynchronizedBehavior"/>. A default private lock
+        /// is fresh per clone; an explicit lock object stays shared.
+        /// </summary>
+        public object Clone() => new SynchronizedBehavior(
+            isShared ? syncRoot : new object(),
+            isShared);
+
+        /// <summary>
+        /// Applies to every method invocation: overlapping calls on the stunt are
+        /// excluded from running concurrently.
+        /// </summary>
+        public bool AppliesTo(IMethodInvocation invocation) => true;
+
+        /// <summary>
+        /// Acquires the lock, invokes <paramref name="next"/>, and releases the lock.
+        /// </summary>
+        public IMethodReturn Execute(IMethodInvocation invocation, ExecuteHandler next)
+        {
+            lock (syncRoot)
+            {
+                return next(invocation, next);
+            }
+        }
+    }
+}