diff --git a/TODO.md b/TODO.md
index b26c925a5..8794b5da6 100644
--- a/TODO.md
+++ b/TODO.md
@@ -95,9 +95,9 @@ A step-by-step checklist for porting MatterControl's design features to a Vue +
 - [x] Add "Select All" (Ctrl+A) shortcut
 
 ### Selection Visualization
-- [ ] Add selection outline effect (OutlinePass or custom)
-- [ ] Add hover highlight on mouseover
-- [ ] Distinguish single vs multi-selection visually
+- [x] Add selection outline effect (OutlinePass or custom)
+- [x] Add hover highlight on mouseover
+- [x] Distinguish single vs multi-selection visually
 
 ### Context Menu
 - [ ] Create right-click context menu component
diff --git a/src/components/Viewport.vue b/src/components/Viewport.vue
index 105b09a4b..0e85f612a 100644
--- a/src/components/Viewport.vue
+++ b/src/components/Viewport.vue
@@ -2,6 +2,10 @@
 import { ref, watch, onMounted, onBeforeUnmount } from 'vue'
 import * as THREE from 'three'
 import { OrbitControls } from 'three/addons/controls/OrbitControls.js'
+import { EffectComposer } from 'three/addons/postprocessing/EffectComposer.js'
+import { RenderPass } from 'three/addons/postprocessing/RenderPass.js'
+import { OutlinePass } from 'three/addons/postprocessing/OutlinePass.js'
+import { OutputPass } from 'three/addons/postprocessing/OutputPass.js'
 import {
   HOME_POSITION,
   HOME_TARGET,
@@ -43,6 +47,10 @@ let axisHelper: THREE.AxesHelper
 let controls: OrbitControls
 let raycaster: THREE.Raycaster
 const mouse = new THREE.Vector2()
+let composer: EffectComposer
+let selectionOutlinePass: OutlinePass
+let hoverOutlinePass: OutlinePass
+let hoveredObject: SceneObject | null = null
 
 function initScene() {
   if (!containerRef.value) return
@@ -91,16 +99,58 @@ function initScene() {
   addLighting()
   addGridHelper()
   addAxisHelper()
-  addTestCube()
 
   raycaster = new THREE.Raycaster()
   sceneStore.setScene(scene)
 
+  addTestCube()
+
+  setupPostProcessing()
   setupMouseHandlers()
   setupResizeObserver()
   animate()
 }
 
+function setupPostProcessing() {
+  if (!containerRef.value) return
+
+  const { clientWidth: width, clientHeight: height } = containerRef.value
+
+  composer = new EffectComposer(renderer)
+
+  const renderPass = new RenderPass(scene, activeCamera)
+  composer.addPass(renderPass)
+
+  // Selection outline - cyan color
+  selectionOutlinePass = new OutlinePass(
+    new THREE.Vector2(width, height),
+    scene,
+    activeCamera
+  )
+  selectionOutlinePass.visibleEdgeColor.set(0x00ffff)
+  selectionOutlinePass.hiddenEdgeColor.set(0x004444)
+  selectionOutlinePass.edgeStrength = 3
+  selectionOutlinePass.edgeThickness = 1
+  selectionOutlinePass.edgeGlow = 0.5
+  composer.addPass(selectionOutlinePass)
+
+  // Hover outline - yellow color, thinner
+  hoverOutlinePass = new OutlinePass(
+    new THREE.Vector2(width, height),
+    scene,
+    activeCamera
+  )
+  hoverOutlinePass.visibleEdgeColor.set(0xffff00)
+  hoverOutlinePass.hiddenEdgeColor.set(0x444400)
+  hoverOutlinePass.edgeStrength = 2
+  hoverOutlinePass.edgeThickness = 1
+  hoverOutlinePass.edgeGlow = 0
+  composer.addPass(hoverOutlinePass)
+
+  const outputPass = new OutputPass()
+  composer.addPass(outputPass)
+}
+
 function addGridHelper() {
   gridHelper = new THREE.GridHelper(10, 10, 0x444444, 0x333333)
   gridHelper.visible = props.showGrid
@@ -147,19 +197,21 @@ function addTestCube() {
   const geometry = new THREE.BoxGeometry(1, 1, 1)
   const material = new THREE.MeshStandardMaterial({ color: 0x4a90d9 })
   const cube = new THREE.Mesh(geometry, material)
-  scene.add(cube)
+  sceneStore.addObject(cube, 'Cube')
 }
 
 function setupMouseHandlers() {
   if (!renderer) return
 
   renderer.domElement.addEventListener('click', onCanvasClick)
+  renderer.domElement.addEventListener('mousemove', onCanvasMouseMove)
 }
 
 function cleanupMouseHandlers() {
   if (!renderer) return
 
   renderer.domElement.removeEventListener('click', onCanvasClick)
+  renderer.domElement.removeEventListener('mousemove', onCanvasMouseMove)
 }
 
 function updateMouseCoordinates(event: MouseEvent) {
@@ -198,6 +250,14 @@ function onCanvasClick(event: MouseEvent) {
   emit('pick', pickedObject, event)
 }
 
+function onCanvasMouseMove(event: MouseEvent) {
+  const newHovered = pickObject(event)
+  if (newHovered !== hoveredObject) {
+    hoveredObject = newHovered
+    updateHoverOutline()
+  }
+}
+
 function setupResizeObserver() {
   if (!containerRef.value) return
 
@@ -218,6 +278,11 @@ function setupResizeObserver() {
         orthographicCamera.updateProjectionMatrix()
 
         renderer.setSize(width, height)
+        composer.setSize(width, height)
+
+        const resolution = new THREE.Vector2(width, height)
+        selectionOutlinePass.resolution = resolution
+        hoverOutlinePass.resolution = resolution
       }
     }
   })
@@ -228,9 +293,34 @@ function setupResizeObserver() {
 function animate() {
   animationFrameId = requestAnimationFrame(animate)
   controls.update()
-  renderer.render(scene, activeCamera)
+  composer.render()
 }
 
+function updateSelectionOutline() {
+  if (!selectionOutlinePass) return
+  selectionOutlinePass.selectedObjects = sceneStore.selectedObjects.map(
+    (obj) => obj.mesh
+  )
+}
+
+function updateHoverOutline() {
+  if (!hoverOutlinePass) return
+  if (hoveredObject && !sceneStore.selectedIds.has(hoveredObject.id)) {
+    hoverOutlinePass.selectedObjects = [hoveredObject.mesh]
+  } else {
+    hoverOutlinePass.selectedObjects = []
+  }
+}
+
+watch(
+  () => sceneStore.selectedObjects,
+  () => {
+    updateSelectionOutline()
+    updateHoverOutline()
+  },
+  { deep: true }
+)
+
 function fitToSelection(objects: THREE.Object3D[]) {
   if (objects.length === 0) return
 
@@ -281,6 +371,10 @@ function setOrthographic(ortho: boolean) {
   // Update controls to use new camera
   controls.object = activeCamera
   controls.update()
+
+  // Update outline passes camera
+  if (selectionOutlinePass) selectionOutlinePass.renderCamera = activeCamera
+  if (hoverOutlinePass) hoverOutlinePass.renderCamera = activeCamera
 }
 
 function toggleProjection() {