diff --git a/src/Nav.js b/src/Nav.js
index fd38ee2..804e771 100644
--- a/src/Nav.js
+++ b/src/Nav.js
@@ -69,33 +69,82 @@ class TagNav extends Component {
   constructor (props) {
     super(props);
 
+    // TODO: fetch this number from config
+    // Number of top tags to display
+    this.numTop = 5;
+
     this.state = {
-      topTags: [
-        'Tag 1',
-        'Tag 2',
-        'Really long tag',
-        'Really really really long tag',
-        'Another tag',
-      ],
+      topTags: [],
     }
   }
 
-  topTags () {
+  fetchTopTags () {
+    fetch('/site.json')
+      .then(resp => resp.json())
+      .then(blob => {
+        let counts = new Map();
+
+        // Count up the tags
+        blob.posts.forEach(post => {
+          post.tags.forEach(tag => {
+            let count = counts.get(tag);
+            if (count === undefined) {
+              count = 0;
+            }
+
+            counts.set(tag, count + 1);
+          });
+        });
+
+        // Sort the tags
+        let tags = [...counts.entries()];
+        tags.sort(function(t1, t2) {
+          let tag1 = t1[0];
+          let tag2 = t2[0];
+          let count1 = t1[1];
+          let count2 = t2[1]
+
+          let diff = count2 - count1;
+
+          if (diff !== 0) {
+            return diff
+          } else {
+            // We have a tie, sort by the strings
+            return tag1.localeCompare(tag2);
+          }
+        });
+
+        // Get just the tag names
+        tags = tags.map(t => t[0]);
+
+        // Get top N
+        tags = tags.slice(0, this.numTop);
+
+        this.setState({topTags: tags});
+      });
+  }
+
+  componentDidMount() {
+    this.fetchTopTags();
   }
 
   render() {
-    // TODO: Populate these tags from the top tags in the post
-    let tags = (
-      <ul>
-        {this.state.topTags.map(tag => (
-          <ListLink icon="tag" key={tag} name={tag} href={"/tag/" + tag} />
-        ))}
-      </ul>
-    );
+    let tags = "";
+    if (this.state.topTags.length > 0) {
+      tags = (
+        <div>
+          <h5 className="nav-header">Top Tags</h5>
+          <ul>
+            {this.state.topTags.map(tag => (
+              <ListLink icon="tag" key={tag} name={tag} href={"/tag/" + tag} />
+            ))}
+          </ul>
+        </div>
+      );
+    }
 
     return (
       <Row>
-        <h5 className="nav-header">Top Tags</h5>
         { tags }
       </Row>
     );