diff --git a/fixtures/blocks/db.json b/fixtures/blocks/db.json
index 8b2067d51a..1d7ec30679 100644
--- a/fixtures/blocks/db.json
+++ b/fixtures/blocks/db.json
@@ -23,37 +23,55 @@
     {
       "id": 1,
       "body": "Hey there",
-      "postId": 1
+      "postId": 1,
+      "userId": 1
     },
     {
       "id": 2,
       "body": "Welcome to the chat",
-      "postId": 1
+      "postId": 1,
+      "userId": 2
     },
     {
       "id": 3,
       "body": "What editor/font are you using?",
-      "postId": 2
+      "postId": 2,
+      "userId": 2
     },
     {
       "id": 4,
       "body": "It's always been hard",
-      "postId": 3
+      "postId": 3,
+      "userId": 1
     },
     {
       "id": 5,
       "body": "It's still easy",
-      "postId": 3
+      "postId": 3,
+      "userId": 2
     }
   ],
   "users": [{
     "id": 1,
-    "name": "Sebastian"
+    "name": "Sebastian",
+    "bioId": 10
   }, {
     "id": 2,
-    "name": "Sophie"
+    "name": "Sophie",
+    "bioId": 20
   }, {
     "id": 3,
-    "name": "Dan"
+    "name": "Dan",
+    "bioId": 30
+  }],
+  "bios": [{
+    "id": 10,
+    "text": "I like European movies"
+  }, {
+    "id": 20,
+    "text": "I like math puzzles"
+  }, {
+    "id": 30,
+    "text": "I like reading twitter"
   }]
 }
diff --git a/fixtures/blocks/src/Router.js b/fixtures/blocks/src/Router.js
index 6d3295004c..19fb76bd8e 100644
--- a/fixtures/blocks/src/Router.js
+++ b/fixtures/blocks/src/Router.js
@@ -24,17 +24,24 @@ const initialState = {
   // TODO: use this for invalidation.
   cache: createCache(),
   url: initialUrl,
+  pendingUrl: initialUrl,
   RootBlock: loadApp(initialUrl),
 };
 
 function reducer(state, action) {
   switch (action.type) {
-    case 'navigate':
+    case 'startNavigation':
+      return {
+        ...state,
+        pendingUrl: action.url,
+      };
+    case 'completeNavigation':
       // TODO: cancel previous fetch?
       return {
-        cache: state.cache,
+        ...state,
         url: action.url,
-        RootBlock: loadApp(action.url),
+        pendingUrl: action.url,
+        RootBlock: action.RootBlock,
       };
     default:
       throw new Error();
@@ -44,7 +51,7 @@ function reducer(state, action) {
 function Router() {
   const [state, dispatch] = useReducer(reducer, initialState);
   const [startTransition, isPending] = useTransition({
-    timeoutMs: 3000,
+    timeoutMs: 1500,
   });
 
   useEffect(() => {
@@ -56,12 +63,16 @@ function Router() {
       startTransition(() => {
         // TODO: Here, There, and Everywhere.
         // TODO: Instant Transitions, somehow.
-        // TODO: Buttons should update immediately.
         dispatch({
-          type: 'navigate',
+          type: 'completeNavigation',
+          RootBlock: loadApp(url),
           url,
         });
       });
+      dispatch({
+        type: 'startNavigation',
+        url,
+      });
     },
     [startTransition]
   );
@@ -76,10 +87,11 @@ function Router() {
 
   const routeContext = useMemo(
     () => ({
+      pendingUrl: state.pendingUrl,
       url: state.url,
       navigate,
     }),
-    [state.url, navigate]
+    [state.url, state.pendingUrl, navigate]
   );
 
   return (
diff --git a/fixtures/blocks/src/client/Link.js b/fixtures/blocks/src/client/Link.js
new file mode 100644
index 0000000000..1f1f535da1
--- /dev/null
+++ b/fixtures/blocks/src/client/Link.js
@@ -0,0 +1,25 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import * as React from 'react';
+import {useRouter} from './RouterContext';
+
+export default function Link({to, children, ...rest}) {
+  const {navigate} = useRouter();
+  return (
+    <a
+      href={to}
+      onClick={e => {
+        e.preventDefault();
+        window.history.pushState(null, null, to);
+        navigate(to);
+      }}
+      {...rest}>
+      {children}
+    </a>
+  );
+}
diff --git a/fixtures/blocks/src/client/Shell.js b/fixtures/blocks/src/client/Shell.js
index e6e1a005f9..07b74ba88a 100644
--- a/fixtures/blocks/src/client/Shell.js
+++ b/fixtures/blocks/src/client/Shell.js
@@ -6,57 +6,25 @@
  */
 
 import * as React from 'react';
-import {useRouter} from './RouterContext';
+import {TabBar, TabLink} from './TabNav';
 
-function TabBar({children}) {
-  return (
-    <div style={{border: '1px solid #aaa', padding: 20, width: 500}}>
-      {children}
-    </div>
-  );
-}
+// TODO: Error Boundaries.
 
-function TabLink({to, children}) {
-  const {url: activeUrl, navigate} = useRouter();
-  const active = activeUrl === to;
-  if (active) {
-    return (
-      <b
-        style={{
-          display: 'inline-block',
-          width: 50,
-          marginRight: 20,
-        }}>
-        {children}
-      </b>
-    );
-  }
+function MainTabNav() {
   return (
-    <a
-      style={{
-        display: 'inline-block',
-        width: 50,
-        marginRight: 20,
-      }}
-      href={to}
-      onClick={e => {
-        e.preventDefault();
-        window.history.pushState(null, null, to);
-        navigate(to);
-      }}>
-      {children}
-    </a>
+    <TabBar>
+      <TabLink to="/">Home</TabLink>
+      <TabLink to="/profile/3" partial={true}>
+        Profile
+      </TabLink>
+    </TabBar>
   );
 }
 
 export default function Shell({children}) {
   return (
     <>
-      <TabBar>
-        <TabLink to="/">Home</TabLink>
-        <TabLink to="/profile">Profile</TabLink>
-      </TabBar>
-      <br />
+      <MainTabNav />
       {children}
     </>
   );
diff --git a/fixtures/blocks/src/client/TabNav.js b/fixtures/blocks/src/client/TabNav.js
new file mode 100644
index 0000000000..142767263f
--- /dev/null
+++ b/fixtures/blocks/src/client/TabNav.js
@@ -0,0 +1,50 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import * as React from 'react';
+import Link from './Link';
+import {useRouter} from './RouterContext';
+
+export function TabBar({children}) {
+  return (
+    <div
+      style={{
+        border: '1px solid #aaa',
+        padding: 20,
+        marginBottom: 20,
+        width: 500,
+      }}>
+      {children}
+    </div>
+  );
+}
+
+export function TabLink({to, partial, children}) {
+  const {pendingUrl: activeUrl} = useRouter();
+  const active = partial ? activeUrl.startsWith(to) : activeUrl === to;
+  if (active) {
+    return (
+      <b
+        style={{
+          display: 'inline-block',
+          marginRight: 20,
+        }}>
+        {children}
+      </b>
+    );
+  }
+  return (
+    <Link
+      style={{
+        display: 'inline-block',
+        marginRight: 20,
+      }}
+      to={to}>
+      {children}
+    </Link>
+  );
+}
diff --git a/fixtures/blocks/src/server/App.block.js b/fixtures/blocks/src/server/App.block.js
index 9347bbe0cf..a02cedd3ce 100644
--- a/fixtures/blocks/src/server/App.block.js
+++ b/fixtures/blocks/src/server/App.block.js
@@ -16,15 +16,13 @@ import loadProfilePage from './ProfilePage.block';
 
 function load(url) {
   let Page;
-  switch (url) {
-    case '/':
-      Page = loadFeedPage();
-      break;
-    case '/profile':
-      Page = loadProfilePage(3);
-      break;
-    default:
-      throw Error('Not found');
+  const segments = url.split('/').filter(Boolean);
+  if (segments.length === 0) {
+    Page = loadFeedPage();
+  } else if (segments[0] === 'profile') {
+    Page = loadProfilePage(Number(segments[1]), segments[2]);
+  } else {
+    throw Error('Not found');
   }
   return {Page};
 }
@@ -33,6 +31,8 @@ function load(url) {
 
 import Shell from '../client/Shell';
 
+// TODO: some notion of key.
+
 function App(props, data) {
   return (
     <Shell>
diff --git a/fixtures/blocks/src/server/Comments.block.js b/fixtures/blocks/src/server/Comments.block.js
index 3e021519fd..fdea53adee 100644
--- a/fixtures/blocks/src/server/Comments.block.js
+++ b/fixtures/blocks/src/server/Comments.block.js
@@ -15,19 +15,25 @@ import {fetch} from 'react-data/fetch';
 
 function load(postId) {
   return {
-    comments: fetch(`/comments?postId=${postId}`).json(),
+    comments: fetch(`/comments?postId=${postId}&_expand=user`).json(),
   };
 }
 
 // Client
 
+import Link from '../client/Link';
+
 function Comments(props, data) {
   return (
     <>
       <h5>Comments</h5>
       <ul>
-        {data.comments.slice(0, 5).map(item => (
-          <li key={item.id}>{item.body}</li>
+        {data.comments.slice(0, 5).map(comment => (
+          <li key={comment.id}>
+            {comment.body}
+            {' • '}
+            <Link to={`/profile/${comment.user.id}`}>{comment.user.name}</Link>
+          </li>
         ))}
       </ul>
     </>
diff --git a/fixtures/blocks/src/server/FeedPage.block.js b/fixtures/blocks/src/server/FeedPage.block.js
index 56ebcaffe2..9fdbb414fe 100644
--- a/fixtures/blocks/src/server/FeedPage.block.js
+++ b/fixtures/blocks/src/server/FeedPage.block.js
@@ -15,7 +15,7 @@ import {fetch} from 'react-data/fetch';
 import PostList from './PostList';
 
 function load(params) {
-  const allPosts = fetch('/posts').json();
+  const allPosts = fetch('/posts?_expand=user').json();
   return {
     posts: <PostList posts={allPosts} />,
   };
diff --git a/fixtures/blocks/src/server/Post.block.js b/fixtures/blocks/src/server/Post.block.js
index 23ce58cbd3..ffc3e63b38 100644
--- a/fixtures/blocks/src/server/Post.block.js
+++ b/fixtures/blocks/src/server/Post.block.js
@@ -21,6 +21,8 @@ function load(postId) {
 
 // Client
 
+import Link from '../client/Link';
+
 function Post(props, data) {
   return (
     <div
@@ -31,7 +33,13 @@ function Post(props, data) {
         padding: 20,
         maxWidth: 500,
       }}>
-      <h4 style={{marginTop: 0}}>{props.post.title}</h4>
+      <h4 style={{marginTop: 0}}>
+        {props.post.title}
+        {' by '}
+        <Link to={`/profile/${props.post.user.id}`}>
+          {props.post.user.name}
+        </Link>
+      </h4>
       <p>{props.post.body}</p>
       <Suspense
         fallback={<h5>Loading comments...</h5>}
diff --git a/fixtures/blocks/src/server/PostList.js b/fixtures/blocks/src/server/PostList.js
index 2baf1da4fc..f479da5eb2 100644
--- a/fixtures/blocks/src/server/PostList.js
+++ b/fixtures/blocks/src/server/PostList.js
@@ -17,7 +17,7 @@ export default function PostList({posts}) {
   return (
     <SuspenseList revealOrder="forwards" tail="collapsed">
       {posts.map(post => {
-        preload(`/comments?postId=${post.id}`);
+        preload(`/comments?postId=${post.id}&_expand=user`);
         const Post = loadPost(post.id);
         return (
           <Suspense key={post.id} fallback={<PostGlimmer />}>
diff --git a/fixtures/blocks/src/server/ProfileBio.block.js b/fixtures/blocks/src/server/ProfileBio.block.js
new file mode 100644
index 0000000000..1142524de2
--- /dev/null
+++ b/fixtures/blocks/src/server/ProfileBio.block.js
@@ -0,0 +1,34 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+/* eslint-disable import/first */
+
+import * as React from 'react';
+import {block} from 'react';
+
+// Server
+
+import {fetch} from 'react-data/fetch';
+
+function load(user) {
+  return {
+    user,
+    bio: fetch(`/bios/${user.bioId}`).json(),
+  };
+}
+
+// Client
+
+function ProfileBio(props, data) {
+  return (
+    <>
+      <h3>{data.user.name}'s Bio</h3>
+      <p>{data.bio.text}</p>
+    </>
+  );
+}
+
+export default block(ProfileBio, load);
diff --git a/fixtures/blocks/src/server/ProfilePage.block.js b/fixtures/blocks/src/server/ProfilePage.block.js
index 6ee61e56b4..668c44b3d7 100644
--- a/fixtures/blocks/src/server/ProfilePage.block.js
+++ b/fixtures/blocks/src/server/ProfilePage.block.js
@@ -12,23 +12,47 @@ import {block, Suspense} from 'react';
 // Server
 
 import {fetch} from 'react-data/fetch';
+import loadProfileBio from './ProfileBio.block';
 import loadProfileTimeline from './ProfileTimeline.block';
 
-function load(userId) {
+function load(userId, tab) {
+  const user = fetch(`/users/${userId}`).json();
+  let Tab;
+  switch (tab) {
+    case 'bio':
+      Tab = loadProfileBio(user);
+      break;
+    default:
+      Tab = loadProfileTimeline(userId);
+      break;
+  }
   return {
-    user: fetch(`/users/${userId}`).json(),
-    ProfileTimeline: loadProfileTimeline(userId),
+    Tab,
+    user,
   };
 }
 
 // Client
 
+import {TabBar, TabLink} from '../client/TabNav';
+
+function ProfileTabNav({userId}) {
+  // TODO: Don't hardcode ID.
+  return (
+    <TabBar>
+      <TabLink to={`/profile/${userId}`}>Timeline</TabLink>
+      <TabLink to={`/profile/${userId}/bio`}>Bio</TabLink>
+    </TabBar>
+  );
+}
+
 function ProfilePage(props, data) {
   return (
     <>
       <h2>{data.user.name}</h2>
-      <Suspense fallback={<h3>Loading Timeline...</h3>}>
-        <data.ProfileTimeline />
+      <ProfileTabNav userId={data.user.id} />
+      <Suspense fallback={<h3>Loading...</h3>}>
+        <data.Tab />
       </Suspense>
     </>
   );
diff --git a/fixtures/blocks/src/server/ProfileTimeline.block.js b/fixtures/blocks/src/server/ProfileTimeline.block.js
index c1c0978a16..8d42eda7a5 100644
--- a/fixtures/blocks/src/server/ProfileTimeline.block.js
+++ b/fixtures/blocks/src/server/ProfileTimeline.block.js
@@ -15,7 +15,7 @@ import {fetch} from 'react-data/fetch';
 import PostList from './PostList';
 
 function load(userId) {
-  const postsByUser = fetch(`/posts?userId=${userId}`).json();
+  const postsByUser = fetch(`/posts?userId=${userId}&_expand=user`).json();
   return {
     posts: <PostList posts={postsByUser} />,
   };
@@ -24,12 +24,7 @@ function load(userId) {
 // Client
 
 function ProfileTimeline(props, data) {
-  return (
-    <>
-      <h3>Timeline</h3>
-      {data.posts}
-    </>
-  );
+  return <>{data.posts}</>;
 }
 
 export default block(ProfileTimeline, load);