diff --git a/21-metal-performance-shaders/README.md b/21-metal-performance-shaders/README.md
new file mode 100644
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@@ -0,0 +1,55 @@
+# Chapter 21: Metal Performance Shaders
+
+As I read this book, I summarize what I think is wrong. If you think my comments are wrong then please let me know. We can dicuss more and update your opinion.
+
+## 3. Secondary rays
+
+In Renderer, in draw(in:), the code for secondary rays is like this:
+
+```
+for _ in 0..<3 {
+  // MARK: generate intersections between rays and model triangles
+  intersector.intersectionDataType = .distancePrimitiveIndexCoordinates
+  intersector.encodeIntersection(commandBuffer: commandBuffer,
+                                 intersectionType: .nearest,
+                                 rayBuffer: rayBuffer, rayBufferOffset: 0,
+                                 intersectionBuffer: intersectionBuffer, intersectionBufferOffset: 0,
+                                 rayCount: width * height,
+                                 accelerationStructure: accelerationStructure)
+  
+  // MARK: shading
+  computeEncoder = commandBuffer.makeComputeCommandEncoder()
+  computeEncoder?.label = "Shading"
+  computeEncoder?.setBuffer(uniformBuffer, offset: uniformBufferOffset, index: 0)
+  computeEncoder?.setBuffer(rayBuffer, offset: 0, index: 1)
+  computeEncoder?.setBuffer(shadowRayBuffer, offset: 0, index: 2)
+  computeEncoder?.setBuffer(intersectionBuffer, offset: 0, index: 3)
+  computeEncoder?.setBuffer(vertexColorBuffer, offset: 0, index: 4)
+  computeEncoder?.setBuffer(vertexNormalBuffer, offset: 0, index: 5)
+  computeEncoder?.setBuffer(randomBuffer, offset: randomBufferOffset, index: 6)
+  computeEncoder?.setTexture(renderTarget, index: 0)
+  computeEncoder?.setComputePipelineState(shadePipelineState!)
+  computeEncoder?.dispatchThreadgroups(threadGroups, threadsPerThreadgroup: threadsPerGroup)
+  computeEncoder?.endEncoding()
+  
+  // MARK: shadows
+  intersector.label = "Shadow Intersector"
+  intersector.intersectionDataType = .distance
+  intersector.encodeIntersection(commandBuffer: commandBuffer,
+                                 intersectionType: .any,
+                                 rayBuffer: shadowRayBuffer, rayBufferOffset: 0,
+                                 intersectionBuffer: intersectionBuffer, intersectionBufferOffset: 0,
+                                 rayCount: width * height,
+                                 accelerationStructure: accelerationStructure)
+  
+  computeEncoder = commandBuffer.makeComputeCommandEncoder()
+  computeEncoder?.label = "Shadows"
+  computeEncoder?.setBuffer(uniformBuffer, offset: uniformBufferOffset, index: 0)
+  computeEncoder?.setBuffer(shadowRayBuffer, offset: 0, index: 1)
+  computeEncoder?.setBuffer(intersectionBuffer, offset: 0, index: 2)
+  computeEncoder?.setTexture(renderTarget, index: 0)
+  computeEncoder?.setComputePipelineState(shadowPipelineState)
+  computeEncoder?.dispatchThreadgroups(threadGroups, threadsPerThreadgroup: threadsPerGroup)
+  computeEncoder?.endEncoding()
+}
+```
\ No newline at end of file
diff --git a/README.md b/README.md
index 9d5c65e..d492018 100644
--- a/README.md
+++ b/README.md
@@ -34,6 +34,7 @@ You can see color images in this book at [here](https://www.raywenderlich.com/bo
  * [Chapter 18: Rendering with Rays](https://github.com/daemyung/metal-by-tutorials/tree/main/18-rendering-with-rays)
  * [Chapter 19: Advanced Shadows](https://github.com/daemyung/metal-by-tutorials/tree/main/19-shadows)
  * [Chapter 20: Advanced Lighting](https://github.com/daemyung/metal-by-tutorials/tree/main/20-advanced-lighting)
+ * [Chapter 21: Metal Performance Shaders](https://github.com/daemyung/metal-by-tutorials/tree/main/21-metal-performance-shaders)
 
 ## Copyright