diff --git a/akka-actor/src/main/scala/akka/actor/Actor.scala b/akka-actor/src/main/scala/akka/actor/Actor.scala
index 2721ccffa0..8fc7df93e5 100644
--- a/akka-actor/src/main/scala/akka/actor/Actor.scala
+++ b/akka-actor/src/main/scala/akka/actor/Actor.scala
@@ -7,7 +7,6 @@ package akka.actor
 import akka.AkkaException
 import scala.reflect.BeanProperty
 import scala.util.control.NoStackTrace
-import scala.collection.immutable.Stack
 import java.util.regex.Pattern
 
 /**
@@ -279,18 +278,14 @@ trait Actor {
    */
   protected[akka] implicit val context: ActorContext = {
     val contextStack = ActorCell.contextStack.get
-
-    def noContextError =
+    if ((contextStack.isEmpty) || (contextStack.head eq null))
       throw new ActorInitializationException(
         "\n\tYou cannot create an instance of [" + getClass.getName + "] explicitly using the constructor (new)." +
           "\n\tYou have to use one of the factory methods to create a new actor. Either use:" +
           "\n\t\t'val actor = context.actorOf(Props[MyActor])'        (to create a supervised child actor from within an actor), or" +
           "\n\t\t'val actor = system.actorOf(Props(new MyActor(..)))' (to create a top level actor from the ActorSystem)")
-
-    if (contextStack.isEmpty) noContextError
     val c = contextStack.head
-    if (c eq null) noContextError
-    ActorCell.contextStack.set(contextStack.push(null))
+    ActorCell.contextStack.set(null :: contextStack)
     c
   }
 
diff --git a/akka-actor/src/main/scala/akka/actor/ActorCell.scala b/akka-actor/src/main/scala/akka/actor/ActorCell.scala
index 9dbe610195..72793513e2 100644
--- a/akka-actor/src/main/scala/akka/actor/ActorCell.scala
+++ b/akka-actor/src/main/scala/akka/actor/ActorCell.scala
@@ -13,7 +13,7 @@ import akka.japi.Procedure
 import java.io.{ NotSerializableException, ObjectOutputStream }
 import akka.serialization.SerializationExtension
 import akka.event.Logging.LogEventException
-import collection.immutable.{ TreeSet, Stack, TreeMap }
+import collection.immutable.{ TreeSet, TreeMap }
 import akka.util.{ Unsafe, Duration, Helpers, NonFatal }
 
 //TODO: everything here for current compatibility - could be limited more
@@ -173,8 +173,8 @@ trait UntypedActorContext extends ActorContext {
  * for! (waves hand)
  */
 private[akka] object ActorCell {
-  val contextStack = new ThreadLocal[Stack[ActorContext]] {
-    override def initialValue = Stack[ActorContext]()
+  val contextStack = new ThreadLocal[List[ActorContext]] {
+    override def initialValue: List[ActorContext] = Nil
   }
 
   final val emptyCancellable: Cancellable = new Cancellable {
@@ -184,7 +184,7 @@ private[akka] object ActorCell {
 
   final val emptyReceiveTimeoutData: (Long, Cancellable) = (-1, emptyCancellable)
 
-  final val behaviorStackPlaceHolder: Stack[Actor.Receive] = Stack.empty.push(Actor.emptyBehavior)
+  final val emptyBehaviorStack: List[Actor.Receive] = Nil
 
   final val emptyActorRefSet: Set[ActorRef] = TreeSet.empty
 
@@ -408,7 +408,7 @@ private[akka] class ActorCell(
 
   var currentMessage: Envelope = _
   var actor: Actor = _
-  private var behaviorStack: Stack[Actor.Receive] = Stack.empty
+  private var behaviorStack: List[Actor.Receive] = emptyBehaviorStack
   @volatile var _mailboxDoNotCallMeDirectly: Mailbox = _ //This must be volatile since it isn't protected by the mailbox status
   var nextNameSequence: Long = 0
   var watching: Set[ActorRef] = emptyActorRefSet
@@ -511,25 +511,21 @@ private[akka] class ActorCell(
 
   //This method is in charge of setting up the contextStack and create a new instance of the Actor
   protected def newActor(): Actor = {
-    contextStack.set(contextStack.get.push(this))
+    contextStack.set(this :: contextStack.get)
     try {
-      import ActorCell.behaviorStackPlaceHolder
-
-      behaviorStack = behaviorStackPlaceHolder
+      behaviorStack = emptyBehaviorStack
       val instance = props.creator.apply()
 
       if (instance eq null)
         throw new ActorInitializationException(self, "Actor instance passed to actorOf can't be 'null'")
 
-      behaviorStack = behaviorStack match {
-        case `behaviorStackPlaceHolder` ⇒ Stack.empty.push(instance.receive)
-        case newBehaviors               ⇒ Stack.empty.push(instance.receive).pushAll(newBehaviors.reverse.drop(1))
-      }
+      // If no becomes were issued, the actors behavior is its receive method
+      behaviorStack = if (behaviorStack.isEmpty) instance.receive :: behaviorStack else behaviorStack
       instance
     } finally {
       val stackAfter = contextStack.get
       if (stackAfter.nonEmpty)
-        contextStack.set(if (stackAfter.head eq null) stackAfter.pop.pop else stackAfter.pop) // pop null marker plus our context
+        contextStack.set(if (stackAfter.head eq null) stackAfter.tail.tail else stackAfter.tail) // pop null marker plus our context
     }
   }
 
@@ -683,10 +679,8 @@ private[akka] class ActorCell(
     }
   }
 
-  def become(behavior: Actor.Receive, discardOld: Boolean = true): Unit = {
-    if (discardOld) unbecome()
-    behaviorStack = behaviorStack.push(behavior)
-  }
+  def become(behavior: Actor.Receive, discardOld: Boolean = true): Unit =
+    behaviorStack = behavior :: (if (discardOld && behaviorStack.nonEmpty) behaviorStack.tail else behaviorStack)
 
   /**
    * UntypedActorContext impl
@@ -701,8 +695,9 @@ private[akka] class ActorCell(
 
   def unbecome(): Unit = {
     val original = behaviorStack
-    val popped = original.pop
-    behaviorStack = if (popped.isEmpty) original else popped
+    behaviorStack =
+      if (original.isEmpty || original.tail.isEmpty) actor.receive :: emptyBehaviorStack
+      else original.tail
   }
 
   def autoReceiveMessage(msg: Envelope): Unit = {
@@ -761,7 +756,7 @@ private[akka] class ActorCell(
         if (system.settings.DebugLifecycle)
           system.eventStream.publish(Debug(self.path.toString, clazz(a), "stopped"))
       } finally {
-        behaviorStack = behaviorStackPlaceHolder
+        behaviorStack = emptyBehaviorStack
         clearActorFields(a)
         actor = null
       }
diff --git a/akka-actor/src/main/scala/akka/actor/ActorSystem.scala b/akka-actor/src/main/scala/akka/actor/ActorSystem.scala
index 0483ccf60f..0d13f2451a 100644
--- a/akka-actor/src/main/scala/akka/actor/ActorSystem.scala
+++ b/akka-actor/src/main/scala/akka/actor/ActorSystem.scala
@@ -13,7 +13,6 @@ import java.io.Closeable
 import akka.dispatch.Await.{ Awaitable, CanAwait }
 import akka.util._
 import akka.util.internal.{ HashedWheelTimer, ConcurrentIdentityHashMap }
-import collection.immutable.Stack
 import java.util.concurrent.{ ThreadFactory, CountDownLatch, TimeoutException, RejectedExecutionException }
 import java.util.concurrent.TimeUnit.MILLISECONDS
 
@@ -685,8 +684,8 @@ private[akka] class ActorSystemImpl(val name: String, applicationConfig: Config,
 
   final class TerminationCallbacks extends Runnable with Awaitable[Unit] {
     private val lock = new ReentrantGuard
-    private var callbacks: Stack[Runnable] = _ //non-volatile since guarded by the lock
-    lock withGuard { callbacks = Stack.empty[Runnable] }
+    private var callbacks: List[Runnable] = _ //non-volatile since guarded by the lock
+    lock withGuard { callbacks = Nil }
 
     private val latch = new CountDownLatch(1)
 
@@ -695,17 +694,17 @@ private[akka] class ActorSystemImpl(val name: String, applicationConfig: Config,
         case 0 ⇒ throw new RejectedExecutionException("Must be called prior to system shutdown.")
         case _ ⇒ lock withGuard {
           if (latch.getCount == 0) throw new RejectedExecutionException("Must be called prior to system shutdown.")
-          else callbacks = callbacks.push(callback)
+          else callbacks ::= callback
         }
       }
     }
 
     final def run(): Unit = lock withGuard {
-      @tailrec def runNext(c: Stack[Runnable]): Stack[Runnable] = c.headOption match {
-        case None ⇒ Stack.empty[Runnable]
-        case Some(callback) ⇒
-          try callback.run() catch { case e ⇒ log.error(e, "Failed to run termination callback, due to [{}]", e.getMessage) }
-          runNext(c.pop)
+      @tailrec def runNext(c: List[Runnable]): List[Runnable] = c match {
+        case Nil ⇒ Nil
+        case callback :: rest ⇒
+          try callback.run() catch { case NonFatal(e) ⇒ log.error(e, "Failed to run termination callback, due to [{}]", e.getMessage) }
+          runNext(rest)
       }
       try { callbacks = runNext(callbacks) } finally latch.countDown()
     }
diff --git a/akka-actor/src/main/scala/akka/actor/Props.scala b/akka-actor/src/main/scala/akka/actor/Props.scala
index fc01a5ba36..82d97f5465 100644
--- a/akka-actor/src/main/scala/akka/actor/Props.scala
+++ b/akka-actor/src/main/scala/akka/actor/Props.scala
@@ -6,7 +6,6 @@ package akka.actor
 
 import akka.dispatch._
 import akka.japi.Creator
-import collection.immutable.Stack
 import akka.routing._
 
 /**
diff --git a/akka-testkit/src/main/scala/akka/testkit/TestActorRef.scala b/akka-testkit/src/main/scala/akka/testkit/TestActorRef.scala
index ed151b6b12..f8efe4e2e5 100644
--- a/akka-testkit/src/main/scala/akka/testkit/TestActorRef.scala
+++ b/akka-testkit/src/main/scala/akka/testkit/TestActorRef.scala
@@ -5,9 +5,7 @@
 package akka.testkit
 
 import akka.actor._
-import akka.util.Duration
 import java.util.concurrent.atomic.AtomicLong
-import scala.collection.immutable.Stack
 import akka.dispatch._
 import akka.pattern.ask
 
diff --git a/akka-testkit/src/main/scala/akka/testkit/TestKit.scala b/akka-testkit/src/main/scala/akka/testkit/TestKit.scala
index c0fb6e5267..4a5a880bb0 100644
--- a/akka-testkit/src/main/scala/akka/testkit/TestKit.scala
+++ b/akka-testkit/src/main/scala/akka/testkit/TestKit.scala
@@ -486,19 +486,21 @@ trait TestKitBase {
 
     @tailrec
     def doit(acc: List[T], count: Int): List[T] = {
-      if (count >= messages) return acc.reverse
-      receiveOne((stop - now) min idle)
-      lastMessage match {
-        case NullMessage ⇒
-          lastMessage = msg
-          acc.reverse
-        case RealMessage(o, _) if (f isDefinedAt o) ⇒
-          msg = lastMessage
-          doit(f(o) :: acc, count + 1)
-        case RealMessage(o, _) ⇒
-          queue.offerFirst(lastMessage)
-          lastMessage = msg
-          acc.reverse
+      if (count >= messages) acc.reverse
+      else {
+        receiveOne((stop - now) min idle)
+        lastMessage match {
+          case NullMessage ⇒
+            lastMessage = msg
+            acc.reverse
+          case RealMessage(o, _) if (f isDefinedAt o) ⇒
+            msg = lastMessage
+            doit(f(o) :: acc, count + 1)
+          case RealMessage(o, _) ⇒
+            queue.offerFirst(lastMessage)
+            lastMessage = msg
+            acc.reverse
+        }
       }
     }