diff --git a/interface/src/Application.cpp b/interface/src/Application.cpp
index f828f05258..d9c5589631 100644
--- a/interface/src/Application.cpp
+++ b/interface/src/Application.cpp
@@ -891,8 +891,11 @@ void Application::paintGL() {
 
     glEnable(GL_LINE_SMOOTH);
     
-    Menu::getInstance()->setIsOptionChecked("First Person", _myAvatar->getBoomLength() <= MyAvatar::ZOOM_MIN);
-    Application::getInstance()->cameraMenuChanged();
+    if (_myCamera.getMode() == CAMERA_MODE_FIRST_PERSON || _myCamera.getMode() == CAMERA_MODE_THIRD_PERSON) {
+        Menu::getInstance()->setIsOptionChecked(MenuOption::FirstPerson, _myAvatar->getBoomLength() <= MyAvatar::ZOOM_MIN);
+        Menu::getInstance()->setIsOptionChecked(MenuOption::ThirdPerson, !(_myAvatar->getBoomLength() <= MyAvatar::ZOOM_MIN));
+        Application::getInstance()->cameraMenuChanged();
+    }
 
     if (_myCamera.getMode() == CAMERA_MODE_FIRST_PERSON) {
         // Always use the default eye position, not the actual head eye position.
@@ -1384,11 +1387,17 @@ void Application::keyPressEvent(QKeyEvent* event) {
                 if (isShifted) {
                     Menu::getInstance()->triggerOption(MenuOption::Mirror);
                 } else {
-                    Menu::getInstance()->triggerOption(MenuOption::FullscreenMirror);
+                    Menu::getInstance()->setIsOptionChecked(MenuOption::FullscreenMirror, !Menu::getInstance()->isOptionChecked(MenuOption::FullscreenMirror));
+                    if (!Menu::getInstance()->isOptionChecked(MenuOption::FullscreenMirror)) {
+                        Menu::getInstance()->setIsOptionChecked(MenuOption::ThirdPerson, true);
+                    }
+                    cameraMenuChanged();
                 }
                 break;
             case Qt::Key_P:
-                 Menu::getInstance()->triggerOption(MenuOption::FirstPerson);
+                 Menu::getInstance()->setIsOptionChecked(MenuOption::FirstPerson, !Menu::getInstance()->isOptionChecked(MenuOption::FirstPerson));
+                 Menu::getInstance()->setIsOptionChecked(MenuOption::ThirdPerson, !Menu::getInstance()->isOptionChecked(MenuOption::FirstPerson));
+                 cameraMenuChanged();
                  break;
             case Qt::Key_Slash:
                 Menu::getInstance()->triggerOption(MenuOption::Stats);
@@ -2351,13 +2360,17 @@ void Application::cameraMenuChanged() {
             _myCamera.setMode(CAMERA_MODE_FIRST_PERSON);
             _myAvatar->setBoomLength(MyAvatar::ZOOM_MIN);
         }
-    } else {
+    } else if (Menu::getInstance()->isOptionChecked(MenuOption::ThirdPerson)) {
         if (_myCamera.getMode() != CAMERA_MODE_THIRD_PERSON) {
             _myCamera.setMode(CAMERA_MODE_THIRD_PERSON);
             if (_myAvatar->getBoomLength() == MyAvatar::ZOOM_MIN) {
                 _myAvatar->setBoomLength(MyAvatar::ZOOM_DEFAULT);
             }
         }
+    } else if (Menu::getInstance()->isOptionChecked(MenuOption::IndependentMode)) {
+        if (_myCamera.getMode() != CAMERA_MODE_INDEPENDENT) {
+            _myCamera.setMode(CAMERA_MODE_INDEPENDENT);
+        }
     }
 }
 
@@ -2451,20 +2464,22 @@ void Application::update(float deltaTime) {
 
     // Transfer the user inputs to the driveKeys
     _myAvatar->clearDriveKeys();
-    if (!_controllerScriptingInterface.areActionsCaptured()) {
-        _myAvatar->setDriveKeys(FWD, _userInputMapper.getActionState(UserInputMapper::LONGITUDINAL_FORWARD));
-        _myAvatar->setDriveKeys(BACK, _userInputMapper.getActionState(UserInputMapper::LONGITUDINAL_BACKWARD));
-        _myAvatar->setDriveKeys(UP, _userInputMapper.getActionState(UserInputMapper::VERTICAL_UP));
-        _myAvatar->setDriveKeys(DOWN, _userInputMapper.getActionState(UserInputMapper::VERTICAL_DOWN));
-        _myAvatar->setDriveKeys(LEFT, _userInputMapper.getActionState(UserInputMapper::LATERAL_LEFT));
-        _myAvatar->setDriveKeys(RIGHT, _userInputMapper.getActionState(UserInputMapper::LATERAL_RIGHT));
-        _myAvatar->setDriveKeys(ROT_UP, _userInputMapper.getActionState(UserInputMapper::PITCH_UP));
-        _myAvatar->setDriveKeys(ROT_DOWN, _userInputMapper.getActionState(UserInputMapper::PITCH_DOWN));
-        _myAvatar->setDriveKeys(ROT_LEFT, _userInputMapper.getActionState(UserInputMapper::YAW_LEFT));
-        _myAvatar->setDriveKeys(ROT_RIGHT, _userInputMapper.getActionState(UserInputMapper::YAW_RIGHT));
+    if (_myCamera.getMode() != CAMERA_MODE_INDEPENDENT) {
+        if (!_controllerScriptingInterface.areActionsCaptured()) {
+            _myAvatar->setDriveKeys(FWD, _userInputMapper.getActionState(UserInputMapper::LONGITUDINAL_FORWARD));
+            _myAvatar->setDriveKeys(BACK, _userInputMapper.getActionState(UserInputMapper::LONGITUDINAL_BACKWARD));
+            _myAvatar->setDriveKeys(UP, _userInputMapper.getActionState(UserInputMapper::VERTICAL_UP));
+            _myAvatar->setDriveKeys(DOWN, _userInputMapper.getActionState(UserInputMapper::VERTICAL_DOWN));
+            _myAvatar->setDriveKeys(LEFT, _userInputMapper.getActionState(UserInputMapper::LATERAL_LEFT));
+            _myAvatar->setDriveKeys(RIGHT, _userInputMapper.getActionState(UserInputMapper::LATERAL_RIGHT));
+            _myAvatar->setDriveKeys(ROT_UP, _userInputMapper.getActionState(UserInputMapper::PITCH_UP));
+            _myAvatar->setDriveKeys(ROT_DOWN, _userInputMapper.getActionState(UserInputMapper::PITCH_DOWN));
+            _myAvatar->setDriveKeys(ROT_LEFT, _userInputMapper.getActionState(UserInputMapper::YAW_LEFT));
+            _myAvatar->setDriveKeys(ROT_RIGHT, _userInputMapper.getActionState(UserInputMapper::YAW_RIGHT));
+        }
+        _myAvatar->setDriveKeys(BOOM_IN, _userInputMapper.getActionState(UserInputMapper::BOOM_IN));
+        _myAvatar->setDriveKeys(BOOM_OUT, _userInputMapper.getActionState(UserInputMapper::BOOM_OUT));
     }
-    _myAvatar->setDriveKeys(BOOM_IN, _userInputMapper.getActionState(UserInputMapper::BOOM_IN));
-    _myAvatar->setDriveKeys(BOOM_OUT, _userInputMapper.getActionState(UserInputMapper::BOOM_OUT));
 
     updateThreads(deltaTime); // If running non-threaded, then give the threads some time to process...
 
@@ -2481,24 +2496,45 @@ void Application::update(float deltaTime) {
 
         _entitySimulation.lock();
         _physicsEngine.deleteObjects(_entitySimulation.getObjectsToDelete());
+        _entitySimulation.unlock();
+
+        _entities.getTree()->lockForWrite();
+        _entitySimulation.lock();
         _physicsEngine.addObjects(_entitySimulation.getObjectsToAdd());
+        _entitySimulation.unlock();
+        _entities.getTree()->unlock();
+
+        _entities.getTree()->lockForWrite();
+        _entitySimulation.lock();
         _physicsEngine.changeObjects(_entitySimulation.getObjectsToChange());
+        _entitySimulation.unlock();
+        _entities.getTree()->unlock();
+
+        _entitySimulation.lock();
         _entitySimulation.applyActionChanges();
         _entitySimulation.unlock();
 
+
         AvatarManager* avatarManager = DependencyManager::get<AvatarManager>().data();
         _physicsEngine.deleteObjects(avatarManager->getObjectsToDelete());
         _physicsEngine.addObjects(avatarManager->getObjectsToAdd());
         _physicsEngine.changeObjects(avatarManager->getObjectsToChange());
 
+        _entities.getTree()->lockForWrite();
         _physicsEngine.stepSimulation();
+        _entities.getTree()->unlock();
 
         if (_physicsEngine.hasOutgoingChanges()) {
+            _entities.getTree()->lockForWrite();
             _entitySimulation.lock();
             _entitySimulation.handleOutgoingChanges(_physicsEngine.getOutgoingChanges(), _physicsEngine.getSessionID());
             _entitySimulation.unlock();
+            _entities.getTree()->unlock();
 
+            _entities.getTree()->lockForWrite();
             avatarManager->handleOutgoingChanges(_physicsEngine.getOutgoingChanges());
+            _entities.getTree()->unlock();
+
             auto collisionEvents = _physicsEngine.getCollisionEvents();
             avatarManager->handleCollisionEvents(collisionEvents);
 
diff --git a/interface/src/Application.h b/interface/src/Application.h
index b126757621..2c78327f02 100644
--- a/interface/src/Application.h
+++ b/interface/src/Application.h
@@ -444,6 +444,8 @@ public slots:
     void notifyPacketVersionMismatch();
 
     void domainConnectionDenied(const QString& reason);
+    
+    void cameraMenuChanged();
 
 private slots:
     void clearDomainOctreeDetails();
@@ -460,7 +462,7 @@ private slots:
     void setFullscreen(bool fullscreen);
     void setEnable3DTVMode(bool enable3DTVMode);
     void setEnableVRMode(bool enableVRMode);
-    void cameraMenuChanged();
+    
     void rotationModeChanged();
 
     glm::vec2 getScaledScreenPoint(glm::vec2 projectedPoint);
diff --git a/interface/src/Camera.cpp b/interface/src/Camera.cpp
index 42ac30dbf3..889f63c2be 100644
--- a/interface/src/Camera.cpp
+++ b/interface/src/Camera.cpp
@@ -87,24 +87,26 @@ PickRay Camera::computePickRay(float x, float y) {
 
 void Camera::setModeString(const QString& mode) {
     CameraMode targetMode = stringToMode(mode);
-    
+        
     switch (targetMode) {
+        case CAMERA_MODE_FIRST_PERSON:
+            Menu::getInstance()->setIsOptionChecked(MenuOption::FirstPerson, true);
+            break;
         case CAMERA_MODE_THIRD_PERSON:
-            Menu::getInstance()->setIsOptionChecked(MenuOption::FullscreenMirror, false);
-            Menu::getInstance()->setIsOptionChecked(MenuOption::FirstPerson, false);
+            Menu::getInstance()->setIsOptionChecked(MenuOption::ThirdPerson, true);
             break;
         case CAMERA_MODE_MIRROR:
             Menu::getInstance()->setIsOptionChecked(MenuOption::FullscreenMirror, true);
-            Menu::getInstance()->setIsOptionChecked(MenuOption::FirstPerson, false);
             break;
         case CAMERA_MODE_INDEPENDENT:
-            Menu::getInstance()->setIsOptionChecked(MenuOption::FullscreenMirror, false);
-            Menu::getInstance()->setIsOptionChecked(MenuOption::FirstPerson, false);
+            Menu::getInstance()->setIsOptionChecked(MenuOption::IndependentMode, true);
             break;
         default:
             break;
     }
     
+    qApp->cameraMenuChanged();
+    
     if (_mode != targetMode) {
         setMode(targetMode);
     }
diff --git a/interface/src/Menu.cpp b/interface/src/Menu.cpp
index 6671320f0c..20d88a59c0 100644
--- a/interface/src/Menu.cpp
+++ b/interface/src/Menu.cpp
@@ -268,9 +268,22 @@ Menu::Menu() {
                                            qApp,
                                            SLOT(setFullscreen(bool)));
 
-    addCheckableActionToQMenuAndActionHash(viewMenu, MenuOption::FirstPerson,
-        0, // QML Qt::Key_P,
-        true, qApp, SLOT(cameraMenuChanged()));
+    MenuWrapper* cameraModeMenu = viewMenu->addMenu("Camera Mode");
+    QActionGroup* cameraModeGroup = new QActionGroup(cameraModeMenu);
+    cameraModeGroup->setExclusive(true);
+    cameraModeGroup->addAction(addCheckableActionToQMenuAndActionHash(cameraModeMenu,
+                                                                      MenuOption::FirstPerson, 0, // QML Qt:: Key_P
+                                                                      false, qApp, SLOT(cameraMenuChanged())));
+    cameraModeGroup->addAction(addCheckableActionToQMenuAndActionHash(cameraModeMenu,
+                                                                      MenuOption::ThirdPerson, 0,
+                                                                      true, qApp, SLOT(cameraMenuChanged())));
+    cameraModeGroup->addAction(addCheckableActionToQMenuAndActionHash(cameraModeMenu,
+                                                                      MenuOption::IndependentMode, 0,
+                                                                      false, qApp, SLOT(cameraMenuChanged())));
+    cameraModeGroup->addAction(addCheckableActionToQMenuAndActionHash(cameraModeMenu,
+                                                                      MenuOption::FullscreenMirror, 0, // QML Qt::Key_H,
+                                                                      false, qApp, SLOT(cameraMenuChanged())));
+
     addCheckableActionToQMenuAndActionHash(viewMenu, MenuOption::Mirror,
         0, //QML Qt::SHIFT | Qt::Key_H,
         true);
diff --git a/interface/src/Menu.h b/interface/src/Menu.h
index be0ac12d0d..0097437107 100644
--- a/interface/src/Menu.h
+++ b/interface/src/Menu.h
@@ -158,6 +158,7 @@ namespace MenuOption {
     const QString CascadedShadows = "Cascaded";
     const QString CachesSize = "RAM Caches Size";
     const QString CalibrateCamera = "Calibrate Camera";
+    const QString CenterPlayerInView = "Center Player In View";
     const QString Chat = "Chat...";
     const QString Collisions = "Collisions";
     const QString Console = "Console...";
@@ -199,11 +200,10 @@ namespace MenuOption {
     const QString FrameTimer = "Show Timer";
     const QString Fullscreen = "Fullscreen";
     const QString FullscreenMirror = "Fullscreen Mirror";
-    const QString CenterPlayerInView = "Center Player In View";
     const QString GlowWhenSpeaking = "Glow When Speaking";
-    const QString NamesAboveHeads = "Names Above Heads";
     const QString HMDTools = "HMD Tools";
     const QString IncreaseAvatarSize = "Increase Avatar Size";
+    const QString IndependentMode = "Independent Mode";
     const QString KeyboardMotorControl = "Enable Keyboard Motor Control";
     const QString LeapMotionOnHMD = "Leap Motion on HMD";
     const QString LoadScript = "Open and Run Script File...";
@@ -217,6 +217,7 @@ namespace MenuOption {
     const QString MuteAudio = "Mute Microphone";
     const QString MuteEnvironment = "Mute Environment";
     const QString MuteFaceTracking = "Mute Face Tracking";
+    const QString NamesAboveHeads = "Names Above Heads";
     const QString NoFaceTracking = "None";
     const QString OctreeStats = "Entity Statistics";
     const QString OnlyDisplayTopTen = "Only Display Top Ten";
@@ -277,6 +278,7 @@ namespace MenuOption {
     const QString StopAllScripts = "Stop All Scripts";
     const QString SuppressShortTimings = "Suppress Timings Less than 10ms";
     const QString TestPing = "Test Ping";
+    const QString ThirdPerson = "Third Person";
     const QString ToolWindow = "Tool Window";
     const QString TransmitterDrive = "Transmitter Drive";
     const QString TurnWithHead = "Turn using Head";
diff --git a/libraries/physics/src/EntityMotionState.cpp b/libraries/physics/src/EntityMotionState.cpp
index 1a516f8872..b6b8a2787c 100644
--- a/libraries/physics/src/EntityMotionState.cpp
+++ b/libraries/physics/src/EntityMotionState.cpp
@@ -19,10 +19,42 @@
 #include "PhysicsHelpers.h"
 #include "PhysicsLogging.h"
 
+#ifdef WANT_DEBUG_ENTITY_TREE_LOCKS
+#include "EntityTree.h"
+#endif
+
 static const float ACCELERATION_EQUIVALENT_EPSILON_RATIO = 0.1f;
 static const quint8 STEPS_TO_DECIDE_BALLISTIC = 4;
 
 
+#ifdef WANT_DEBUG_ENTITY_TREE_LOCKS
+bool EntityMotionState::entityTreeIsLocked() const {
+    EntityTreeElement* element = _entity ? _entity->getElement() : nullptr;
+    EntityTree* tree = element ? element->getTree() : nullptr;
+    if (tree) {
+        bool readSuccess = tree->tryLockForRead();
+        if (readSuccess) {
+            tree->unlock();
+        }
+        bool writeSuccess = tree->tryLockForWrite();
+        if (writeSuccess) {
+            tree->unlock();
+        }
+        if (readSuccess && writeSuccess) {
+            return false;  // if we can take either kind of lock, there was no tree lock.
+        }
+        return true; // either read or write failed, so there is some lock in place.
+    } else {
+        return true;
+    }
+}
+#else
+bool entityTreeIsLocked() {
+    return true;
+}
+#endif
+
+
 EntityMotionState::EntityMotionState(btCollisionShape* shape, EntityItemPointer entity) :
     ObjectMotionState(shape),
     _entity(entity),
@@ -46,6 +78,7 @@ EntityMotionState::EntityMotionState(btCollisionShape* shape, EntityItemPointer
 {
     _type = MOTIONSTATE_TYPE_ENTITY;
     assert(_entity != nullptr);
+    assert(entityTreeIsLocked());
     setMass(_entity->computeMass());
 }
 
@@ -55,6 +88,7 @@ EntityMotionState::~EntityMotionState() {
 }
 
 void EntityMotionState::updateServerPhysicsVariables() {
+    assert(entityTreeIsLocked());
     _serverPosition = _entity->getPosition();
     _serverRotation = _entity->getRotation();
     _serverVelocity = _entity->getVelocity();
@@ -64,6 +98,7 @@ void EntityMotionState::updateServerPhysicsVariables() {
 
 // virtual
 void EntityMotionState::handleEasyChanges(uint32_t flags) {
+    assert(entityTreeIsLocked());
     updateServerPhysicsVariables();
     ObjectMotionState::handleEasyChanges(flags);
     if (flags & EntityItem::DIRTY_SIMULATOR_ID) {
@@ -105,6 +140,7 @@ MotionType EntityMotionState::computeObjectMotionType() const {
     if (!_entity) {
         return MOTION_TYPE_STATIC;
     }
+    assert(entityTreeIsLocked());
     if (_entity->getCollisionsWillMove()) {
         return MOTION_TYPE_DYNAMIC;
     }
@@ -112,6 +148,7 @@ MotionType EntityMotionState::computeObjectMotionType() const {
 }
 
 bool EntityMotionState::isMoving() const {
+    assert(entityTreeIsLocked());
     return _entity && _entity->isMoving();
 }
 
@@ -124,6 +161,7 @@ void EntityMotionState::getWorldTransform(btTransform& worldTrans) const {
     if (!_entity) {
         return;
     }
+    assert(entityTreeIsLocked());
     if (_motionType == MOTION_TYPE_KINEMATIC) {
         // This is physical kinematic motion which steps strictly by the subframe count
         // of the physics simulation.
@@ -144,6 +182,7 @@ void EntityMotionState::setWorldTransform(const btTransform& worldTrans) {
     if (!_entity) {
         return;
     }
+    assert(entityTreeIsLocked());
     measureBodyAcceleration();
     _entity->setPosition(bulletToGLM(worldTrans.getOrigin()) + ObjectMotionState::getWorldOffset());
     _entity->setRotation(bulletToGLM(worldTrans.getRotation()));
@@ -168,9 +207,12 @@ void EntityMotionState::setWorldTransform(const btTransform& worldTrans) {
     #ifdef WANT_DEBUG
         quint64 now = usecTimestampNow();
         qCDebug(physics) << "EntityMotionState::setWorldTransform()... changed entity:" << _entity->getEntityItemID();
-        qCDebug(physics) << "       last edited:" << _entity->getLastEdited() << formatUsecTime(now - _entity->getLastEdited()) << "ago";
-        qCDebug(physics) << "    last simulated:" << _entity->getLastSimulated() << formatUsecTime(now - _entity->getLastSimulated()) << "ago";
-        qCDebug(physics) << "      last updated:" << _entity->getLastUpdated() << formatUsecTime(now - _entity->getLastUpdated()) << "ago";
+        qCDebug(physics) << "       last edited:" << _entity->getLastEdited()
+                         << formatUsecTime(now - _entity->getLastEdited()) << "ago";
+        qCDebug(physics) << "    last simulated:" << _entity->getLastSimulated()
+                         << formatUsecTime(now - _entity->getLastSimulated()) << "ago";
+        qCDebug(physics) << "      last updated:" << _entity->getLastUpdated()
+                         << formatUsecTime(now - _entity->getLastUpdated()) << "ago";
     #endif
 }
 
@@ -178,16 +220,18 @@ void EntityMotionState::setWorldTransform(const btTransform& worldTrans) {
 btCollisionShape* EntityMotionState::computeNewShape() {
     if (_entity) {
         ShapeInfo shapeInfo;
+        assert(entityTreeIsLocked());
         _entity->computeShapeInfo(shapeInfo);
         return getShapeManager()->getShape(shapeInfo);
     }
     return nullptr;
 }
 
-bool EntityMotionState::isCandidateForOwnership(const QUuid& sessionID) const { 
+bool EntityMotionState::isCandidateForOwnership(const QUuid& sessionID) const {
     if (!_body || !_entity) {
         return false;
     }
+    assert(entityTreeIsLocked());
     return _candidateForOwnership || sessionID == _entity->getSimulatorID();
 }
 
@@ -204,7 +248,7 @@ bool EntityMotionState::remoteSimulationOutOfSync(uint32_t simulationStep) {
         _sentActive = false;
         return false;
     }
-    
+
     #ifdef WANT_DEBUG
     glm::vec3 wasPosition = _serverPosition;
     glm::quat wasRotation = _serverRotation;
@@ -217,7 +261,7 @@ bool EntityMotionState::remoteSimulationOutOfSync(uint32_t simulationStep) {
     const float INACTIVE_UPDATE_PERIOD = 0.5f;
     if (!_sentActive) {
         // we resend the inactive update every INACTIVE_UPDATE_PERIOD
-        // until it is removed from the outgoing updates 
+        // until it is removed from the outgoing updates
         // (which happens when we don't own the simulation and it isn't touching our simulation)
         return (dt > INACTIVE_UPDATE_PERIOD);
     }
@@ -235,10 +279,10 @@ bool EntityMotionState::remoteSimulationOutOfSync(uint32_t simulationStep) {
         _serverPosition += dt * _serverVelocity;
     }
 
-    // Else we measure the error between current and extrapolated transform (according to expected behavior 
+    // Else we measure the error between current and extrapolated transform (according to expected behavior
     // of remote EntitySimulation) and return true if the error is significant.
 
-    // NOTE: math is done in the simulation-frame, which is NOT necessarily the same as the world-frame 
+    // NOTE: math is done in the simulation-frame, which is NOT necessarily the same as the world-frame
     // due to _worldOffset.
     // TODO: compensate for _worldOffset offset here
 
@@ -246,7 +290,7 @@ bool EntityMotionState::remoteSimulationOutOfSync(uint32_t simulationStep) {
 
     btTransform worldTrans = _body->getWorldTransform();
     glm::vec3 position = bulletToGLM(worldTrans.getOrigin());
-    
+
     float dx2 = glm::distance2(position, _serverPosition);
 
     const float MAX_POSITION_ERROR_SQUARED = 0.000004f; //  Sqrt() - corresponds to 2 millimeters
@@ -262,13 +306,13 @@ bool EntityMotionState::remoteSimulationOutOfSync(uint32_t simulationStep) {
 
         return true;
     }
-    
+
     if (glm::length2(_serverAngularVelocity) > 0.0f) {
         // compute rotation error
         float attenuation = powf(1.0f - _body->getAngularDamping(), dt);
         _serverAngularVelocity *= attenuation;
-   
-        // Bullet caps the effective rotation velocity inside its rotation integration step, therefore 
+
+        // Bullet caps the effective rotation velocity inside its rotation integration step, therefore
         // we must integrate with the same algorithm and timestep in order achieve similar results.
         for (int i = 0; i < numSteps; ++i) {
             _serverRotation = glm::normalize(computeBulletRotationStep(_serverAngularVelocity, PHYSICS_ENGINE_FIXED_SUBSTEP) * _serverRotation);
@@ -280,7 +324,7 @@ bool EntityMotionState::remoteSimulationOutOfSync(uint32_t simulationStep) {
     #ifdef WANT_DEBUG
         if ((fabsf(glm::dot(actualRotation, _serverRotation)) < MIN_ROTATION_DOT)) {
             qCDebug(physics) << ".... ((fabsf(glm::dot(actualRotation, _serverRotation)) < MIN_ROTATION_DOT)) ....";
-        
+
             qCDebug(physics) << "wasAngularVelocity:" << wasAngularVelocity;
             qCDebug(physics) << "_serverAngularVelocity:" << _serverAngularVelocity;
 
@@ -297,10 +341,11 @@ bool EntityMotionState::remoteSimulationOutOfSync(uint32_t simulationStep) {
 }
 
 bool EntityMotionState::shouldSendUpdate(uint32_t simulationStep, const QUuid& sessionID) {
-    // NOTE: we expect _entity and _body to be valid in this context, since shouldSendUpdate() is only called 
-    // after doesNotNeedToSendUpdate() returns false and that call should return 'true' if _entity or _body are NULL. 
+    // NOTE: we expect _entity and _body to be valid in this context, since shouldSendUpdate() is only called
+    // after doesNotNeedToSendUpdate() returns false and that call should return 'true' if _entity or _body are NULL.
     assert(_entity);
     assert(_body);
+    assert(entityTreeIsLocked());
 
     if (!remoteSimulationOutOfSync(simulationStep)) {
         _candidateForOwnership = false;
@@ -330,6 +375,7 @@ bool EntityMotionState::shouldSendUpdate(uint32_t simulationStep, const QUuid& s
 
 void EntityMotionState::sendUpdate(OctreeEditPacketSender* packetSender, const QUuid& sessionID, uint32_t step) {
     assert(_entity);
+    assert(entityTreeIsLocked());
 
     bool active = _body->isActive();
     if (!active) {
@@ -439,17 +485,18 @@ void EntityMotionState::sendUpdate(OctreeEditPacketSender* packetSender, const Q
     _lastStep = step;
 }
 
-uint32_t EntityMotionState::getAndClearIncomingDirtyFlags() { 
+uint32_t EntityMotionState::getAndClearIncomingDirtyFlags() {
+    assert(entityTreeIsLocked());
     uint32_t dirtyFlags = 0;
     if (_body && _entity) {
-        dirtyFlags = _entity->getDirtyFlags(); 
+        dirtyFlags = _entity->getDirtyFlags();
         _entity->clearDirtyFlags();
         // we add DIRTY_MOTION_TYPE if the body's motion type disagrees with entity velocity settings
         int bodyFlags = _body->getCollisionFlags();
         bool isMoving = _entity->isMoving();
         if (((bodyFlags & btCollisionObject::CF_STATIC_OBJECT) && isMoving) ||
                 (bodyFlags & btCollisionObject::CF_KINEMATIC_OBJECT && !isMoving)) {
-            dirtyFlags |= EntityItem::DIRTY_MOTION_TYPE; 
+            dirtyFlags |= EntityItem::DIRTY_MOTION_TYPE;
         }
     }
     return dirtyFlags;
@@ -459,6 +506,7 @@ uint32_t EntityMotionState::getAndClearIncomingDirtyFlags() {
 // virtual
 QUuid EntityMotionState::getSimulatorID() const {
     if (_entity) {
+        assert(entityTreeIsLocked());
         return _entity->getSimulatorID();
     }
     return QUuid();
@@ -473,12 +521,12 @@ void EntityMotionState::bump() {
 void EntityMotionState::resetMeasuredBodyAcceleration() {
     _lastMeasureStep = ObjectMotionState::getWorldSimulationStep();
     if (_body) {
-        _lastVelocity = bulletToGLM(_body->getLinearVelocity());                                                        
+        _lastVelocity = bulletToGLM(_body->getLinearVelocity());
     } else {
         _lastVelocity = glm::vec3(0.0f);
     }
     _measuredAcceleration = glm::vec3(0.0f);
-} 
+}
 
 void EntityMotionState::measureBodyAcceleration() {
     // try to manually measure the true acceleration of the object
@@ -508,7 +556,7 @@ glm::vec3 EntityMotionState::getObjectLinearVelocityChange() const {
     return _measuredAcceleration * _measuredDeltaTime;
 }
 
-// virtual 
+// virtual
 void EntityMotionState::setMotionType(MotionType motionType) {
     ObjectMotionState::setMotionType(motionType);
     resetMeasuredBodyAcceleration();
@@ -518,12 +566,13 @@ void EntityMotionState::setMotionType(MotionType motionType) {
 // virtual
 QString EntityMotionState::getName() {
     if (_entity) {
+        assert(entityTreeIsLocked());
         return _entity->getName();
     }
     return "";
 }
 
-// virtual 
+// virtual
 int16_t EntityMotionState::computeCollisionGroup() {
     switch (computeObjectMotionType()){
         case MOTION_TYPE_STATIC:
diff --git a/libraries/physics/src/EntityMotionState.h b/libraries/physics/src/EntityMotionState.h
index 2d732f8ee0..4f777a4575 100644
--- a/libraries/physics/src/EntityMotionState.h
+++ b/libraries/physics/src/EntityMotionState.h
@@ -83,6 +83,10 @@ public:
     friend class PhysicalEntitySimulation;
 
 protected:
+    #ifdef WANT_DEBUG_ENTITY_TREE_LOCKS
+    bool entityTreeIsLocked() const;
+    #endif
+
     virtual btCollisionShape* computeNewShape();
     virtual void clearObjectBackPointer();
     virtual void setMotionType(MotionType motionType);