diff --git a/Go/A Tour of Go/Exercise_Web_Crawler.go b/Go/A Tour of Go/Exercise_Web_Crawler.go
new file mode 100644
index 0000000..f614f9b
--- /dev/null
+++ b/Go/A Tour of Go/Exercise_Web_Crawler.go	
@@ -0,0 +1,102 @@
+package main
+
+import (
+	"fmt"
+	"sync"
+	"time"
+)
+
+type Fetcher interface {
+	// Fetch 返回 URL 的 body 内容，并且将在这个页面上找到的 URL 放到一个 slice 中。
+	Fetch(url string) (body string, urls []string, err error)
+}
+
+type URLCache struct {
+	urls map[string]bool
+	mux sync.Mutex
+}
+
+var g_urlCache URLCache = URLCache{
+	urls: map[string]bool{"https://golang.org/": true},
+}
+
+// Crawl 使用 fetcher 从某个 URL 开始递归的爬取页面，直到达到最大深度。
+func Crawl(url string, depth int, fetcher Fetcher) {
+	// TODO: 并行的抓取 URL。
+	// TODO: 不重复抓取页面。
+        // 下面并没有实现上面两种情况：
+	if depth <= 0 {
+		return
+	}
+	body, urls, err := fetcher.Fetch(url)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Printf("found: %s %q\n", url, body)
+	g_urlCache.mux.Lock()
+	for _, u := range urls {
+		_, ok := g_urlCache.urls[u]
+		// fmt.Printf("current u is: %s, ok is: %v\n", u, ok)
+		if !ok {
+			g_urlCache.urls[u] = true
+			go Crawl(u, depth-1, fetcher)
+		}
+	}
+	g_urlCache.mux.Unlock()
+	return
+}
+
+func main() {
+	Crawl("https://golang.org/", 4, fetcher)
+	time.Sleep(5 * time.Second)
+}
+
+// fakeFetcher 是返回若干结果的 Fetcher。
+type fakeFetcher map[string]*fakeResult
+
+type fakeResult struct {
+	body string
+	urls []string
+}
+
+func (f fakeFetcher) Fetch(url string) (string, []string, error) {
+	if res, ok := f[url]; ok {
+		return res.body, res.urls, nil
+	}
+	return "", nil, fmt.Errorf("not found: %s", url)
+}
+
+// fetcher 是填充后的 fakeFetcher。
+var fetcher = fakeFetcher{
+	"https://golang.org/": &fakeResult{
+		"The Go Programming Language",
+		[]string{
+			"https://golang.org/pkg/",
+			"https://golang.org/cmd/",
+		},
+	},
+	"https://golang.org/pkg/": &fakeResult{
+		"Packages",
+		[]string{
+			"https://golang.org/",
+			"https://golang.org/cmd/",
+			"https://golang.org/pkg/fmt/",
+			"https://golang.org/pkg/os/",
+		},
+	},
+	"https://golang.org/pkg/fmt/": &fakeResult{
+		"Package fmt",
+		[]string{
+			"https://golang.org/",
+			"https://golang.org/pkg/",
+		},
+	},
+	"https://golang.org/pkg/os/": &fakeResult{
+		"Package os",
+		[]string{
+			"https://golang.org/",
+			"https://golang.org/pkg/",
+		},
+	},
+}
diff --git a/Go/A Tour of Go/Exercise_Web_Crawler_waitGroup.go b/Go/A Tour of Go/Exercise_Web_Crawler_waitGroup.go
new file mode 100644
index 0000000..59a2027
--- /dev/null
+++ b/Go/A Tour of Go/Exercise_Web_Crawler_waitGroup.go	
@@ -0,0 +1,119 @@
+package main
+
+import (
+	"fmt"
+	"sync"
+)
+
+type urlsCache struct {
+	mux   sync.Mutex
+	urls map[string]bool
+}
+
+func (cache *urlsCache) setVisited(name string) bool {
+	cache.mux.Lock()
+	defer cache.mux.Unlock()
+
+	if cache.urls[name] {	// this url is in map, skip
+		return true
+	}
+	cache.urls[name] = true	// mark it as used.
+	return false	// this url has never been searched, will recursively call crawler
+}
+
+var cacheInstance = urlsCache{urls: make(map[string]bool)}
+
+// Fetcher interface
+type Fetcher interface {
+	// Fetch returns the body of URL and
+	// a slice of URLs found on that page.
+	Fetch(url string) (body string, urls []string, err error)
+}
+
+func crawlInner(url string, depth int, fetcher Fetcher, wg *sync.WaitGroup) {
+	defer wg.Done()
+	if depth <= 0 {
+		return
+	}
+
+	if cacheInstance.setVisited(url) {	// this url is in map, skip
+		return
+	}
+
+	body, urls, err := fetcher.Fetch(url)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Printf("found: %s %q\n", url, body)
+	for _, u := range urls {
+		wg.Add(1)
+		go crawlInner(u, depth-1, fetcher, wg)
+	}
+	return
+}
+
+// Crawl uses fetcher to recursively crawl
+// pages starting with url, to a maximum of depth.
+func Crawl(url string, depth int, fetcher Fetcher) {
+	waitGroup := &sync.WaitGroup{}
+
+	waitGroup.Add(1)
+
+	go crawlInner(url, depth, fetcher, waitGroup)
+
+	waitGroup.Wait()
+}
+
+func main() {
+	Crawl("https://golang.org/", 4, fetcher)
+}
+
+// fakeFetcher is Fetcher that returns canned results.
+type fakeFetcher map[string]*fakeResult
+
+type fakeResult struct {
+	body string
+	urls []string
+}
+
+func (f fakeFetcher) Fetch(url string) (string, []string, error) {
+	if res, ok := f[url]; ok {
+		return res.body, res.urls, nil
+	}
+	return "", nil, fmt.Errorf("not found: %s", url)
+}
+
+// fetcher is a populated fakeFetcher.
+var fetcher = fakeFetcher{
+	"https://golang.org/": &fakeResult{
+		"The Go Programming Language",
+		[]string{
+			"https://golang.org/pkg/",
+			"https://golang.org/cmd/",
+		},
+	},
+	"https://golang.org/pkg/": &fakeResult{
+		"Packages",
+		[]string{
+			"https://golang.org/",
+			"https://golang.org/cmd/",
+			"https://golang.org/pkg/fmt/",
+			"https://golang.org/pkg/os/",
+		},
+	},
+	"https://golang.org/pkg/fmt/": &fakeResult{
+		"Package fmt",
+		[]string{
+			"https://golang.org/",
+			"https://golang.org/pkg/",
+		},
+	},
+	"https://golang.org/pkg/os/": &fakeResult{
+		"Package os",
+		[]string{
+			"https://golang.org/",
+			"https://golang.org/pkg/",
+		},
+	},
+}
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