diff --git a/src/renderers/dom/fiber/ReactDOMFiberEntry.js b/src/renderers/dom/fiber/ReactDOMFiberEntry.js
index 1bdb813b1ef..d319b9d2d87 100644
--- a/src/renderers/dom/fiber/ReactDOMFiberEntry.js
+++ b/src/renderers/dom/fiber/ReactDOMFiberEntry.js
@@ -732,7 +732,7 @@ function renderSubtreeIntoContainer(
         );
       }
     }
-    const newRoot = DOMRenderer.createContainer(container);
+    const newRoot = DOMRenderer.createContainer(container, shouldHydrate);
     root = container._reactRootContainer = newRoot;
     // Initial mount should not be batched.
     DOMRenderer.unbatchedUpdates(() => {
@@ -757,7 +757,39 @@ function createPortal(
   return ReactPortal.createPortal(children, container, null, key);
 }
 
+type ReactRootNode = {
+  render(children: ReactNodeList, callback: ?() => mixed): void,
+  unmount(callback: ?() => mixed): void,
+
+  _reactRootContainer: *,
+};
+
+type RootOptions = {
+  hydrate?: boolean,
+};
+
+function ReactRoot(container: Container, hydrate: boolean) {
+  const root = DOMRenderer.createContainer(container, hydrate);
+  this._reactRootContainer = root;
+}
+ReactRoot.prototype.render = function(
+  children: ReactNodeList,
+  callback: ?() => mixed,
+): void {
+  const root = this._reactRootContainer;
+  DOMRenderer.updateContainer(children, root, null, callback);
+};
+ReactRoot.prototype.unmount = function(callback) {
+  const root = this._reactRootContainer;
+  DOMRenderer.updateContainer(null, root, null, callback);
+};
+
 var ReactDOMFiber = {
+  createRoot(container: DOMContainer, options?: RootOptions): ReactRootNode {
+    const hydrate = options != null && options.hydrate === true;
+    return new ReactRoot(container, hydrate);
+  },
+
   createPortal,
 
   findDOMNode(
diff --git a/src/renderers/dom/shared/__tests__/ReactDOMRoot-test.js b/src/renderers/dom/shared/__tests__/ReactDOMRoot-test.js
new file mode 100644
index 00000000000..c4a327b116e
--- /dev/null
+++ b/src/renderers/dom/shared/__tests__/ReactDOMRoot-test.js
@@ -0,0 +1,71 @@
+/**
+ * Copyright (c) 2013-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ */
+
+'use strict';
+
+var React = require('react');
+var ReactDOM = require('react-dom');
+var ReactDOMServer = require('react-dom/server');
+
+describe('ReactDOMRoot', () => {
+  let container;
+
+  beforeEach(() => {
+    container = document.createElement('div');
+  });
+
+  it('renders children', () => {
+    const root = ReactDOM.createRoot(container);
+    root.render(<div>Hi</div>);
+    expect(container.textContent).toEqual('Hi');
+  });
+
+  it('unmounts children', () => {
+    const root = ReactDOM.createRoot(container);
+    root.render(<div>Hi</div>);
+    expect(container.textContent).toEqual('Hi');
+    root.unmount();
+    expect(container.textContent).toEqual('');
+  });
+
+  it('supports hydration', async () => {
+    const markup = await new Promise(resolve =>
+      resolve(
+        ReactDOMServer.renderToString(<div><span className="extra" /></div>),
+      ),
+    );
+
+    spyOn(console, 'error');
+
+    // Does not hydrate by default
+    const container1 = document.createElement('div');
+    container1.innerHTML = markup;
+    const root1 = ReactDOM.createRoot(container1);
+    root1.render(<div><span /></div>);
+    expect(console.error.calls.count()).toBe(0);
+
+    // Accepts `hydrate` option
+    const container2 = document.createElement('div');
+    container2.innerHTML = markup;
+    const root2 = ReactDOM.createRoot(container2, {hydrate: true});
+    root2.render(<div><span /></div>);
+    expect(console.error.calls.count()).toBe(1);
+    expect(console.error.calls.argsFor(0)[0]).toMatch('Extra attributes');
+  });
+
+  it('does not clear existing children', async () => {
+    spyOn(console, 'error');
+    container.innerHTML = '<div>a</div><div>b</div>';
+    const root = ReactDOM.createRoot(container);
+    root.render(<div><span>c</span><span>d</span></div>);
+    expect(container.textContent).toEqual('abcd');
+    root.render(<div><span>d</span><span>c</span></div>);
+    expect(container.textContent).toEqual('abdc');
+  });
+});
diff --git a/src/renderers/dom/shared/__tests__/ReactDOMServerIntegration-test.js b/src/renderers/dom/shared/__tests__/ReactDOMServerIntegration-test.js
index 15ea880a94c..794a4898b6b 100644
--- a/src/renderers/dom/shared/__tests__/ReactDOMServerIntegration-test.js
+++ b/src/renderers/dom/shared/__tests__/ReactDOMServerIntegration-test.js
@@ -2638,6 +2638,12 @@ describe('ReactDOMServerIntegration', () => {
       it('should error reconnecting different element types', () =>
         expectMarkupMismatch(<div />, <span />));
 
+      it('should error reconnecting fewer root children', () =>
+        expectMarkupMismatch(<span key="a" />, [
+          <span key="a" />,
+          <span key="b" />,
+        ]));
+
       it('should error reconnecting missing attributes', () =>
         expectMarkupMismatch(<div id="foo" />, <div />));
 
diff --git a/src/renderers/dom/shared/__tests__/ReactRenderDocument-test.js b/src/renderers/dom/shared/__tests__/ReactRenderDocument-test.js
index a68ba409e8e..1c86f2b5485 100644
--- a/src/renderers/dom/shared/__tests__/ReactRenderDocument-test.js
+++ b/src/renderers/dom/shared/__tests__/ReactRenderDocument-test.js
@@ -396,7 +396,7 @@ describe('rendering React components at document', () => {
       expect(container.textContent).toBe('parsnip');
       expectDev(console.error.calls.count()).toBe(1);
       expectDev(console.error.calls.argsFor(0)[0]).toContain(
-        'Did not expect server HTML to contain the text node "potato" in <div>.',
+        'Expected server HTML to contain a matching <div> in <div>.',
       );
     });
 
diff --git a/src/renderers/native-cs/ReactNativeCSFiberEntry.js b/src/renderers/native-cs/ReactNativeCSFiberEntry.js
index 72ceb060c70..66c7cdc0adb 100644
--- a/src/renderers/native-cs/ReactNativeCSFiberEntry.js
+++ b/src/renderers/native-cs/ReactNativeCSFiberEntry.js
@@ -194,7 +194,7 @@ const ReactNativeCSFiber: ReactNativeCSType = {
     if (!root) {
       // TODO (bvaughn): If we decide to keep the wrapper component,
       // We could create a wrapper for containerTag as well to reduce special casing.
-      root = ReactNativeCSFiberRenderer.createContainer(containerTag);
+      root = ReactNativeCSFiberRenderer.createContainer(containerTag, false);
       roots.set(containerTag, root);
     }
     ReactNativeCSFiberRenderer.updateContainer(element, root, null, callback);
diff --git a/src/renderers/native-rt/ReactNativeRTFiberEntry.js b/src/renderers/native-rt/ReactNativeRTFiberEntry.js
index a367e32336c..684b6a7154c 100644
--- a/src/renderers/native-rt/ReactNativeRTFiberEntry.js
+++ b/src/renderers/native-rt/ReactNativeRTFiberEntry.js
@@ -53,7 +53,7 @@ const ReactNativeRTFiber: ReactNativeRTType = {
     if (!root) {
       // TODO (bvaughn): If we decide to keep the wrapper component,
       // We could create a wrapper for containerTag as well to reduce special casing.
-      root = ReactNativeRTFiberRenderer.createContainer(containerTag);
+      root = ReactNativeRTFiberRenderer.createContainer(containerTag, false);
       roots.set(containerTag, root);
     }
     ReactNativeRTFiberRenderer.updateContainer(element, root, null, callback);
diff --git a/src/renderers/native/ReactNativeFiberEntry.js b/src/renderers/native/ReactNativeFiberEntry.js
index b5ba64dbd3f..e149c74a7fa 100644
--- a/src/renderers/native/ReactNativeFiberEntry.js
+++ b/src/renderers/native/ReactNativeFiberEntry.js
@@ -52,7 +52,7 @@ const ReactNativeFiber: ReactNativeType = {
     if (!root) {
       // TODO (bvaughn): If we decide to keep the wrapper component,
       // We could create a wrapper for containerTag as well to reduce special casing.
-      root = ReactNativeFiberRenderer.createContainer(containerTag);
+      root = ReactNativeFiberRenderer.createContainer(containerTag, false);
       roots.set(containerTag, root);
     }
     ReactNativeFiberRenderer.updateContainer(element, root, null, callback);
diff --git a/src/renderers/noop/ReactNoopEntry.js b/src/renderers/noop/ReactNoopEntry.js
index a1b7cf099a0..89bfc33c2d7 100644
--- a/src/renderers/noop/ReactNoopEntry.js
+++ b/src/renderers/noop/ReactNoopEntry.js
@@ -269,7 +269,7 @@ var ReactNoop = {
     if (!root) {
       const container = {rootID: rootID, children: []};
       rootContainers.set(rootID, container);
-      root = NoopRenderer.createContainer(container);
+      root = NoopRenderer.createContainer(container, false);
       roots.set(rootID, root);
     }
     NoopRenderer.updateContainer(element, root, null, callback);
diff --git a/src/renderers/shared/fiber/ReactFiberBeginWork.js b/src/renderers/shared/fiber/ReactFiberBeginWork.js
index 59499e01ca0..508a41a8622 100644
--- a/src/renderers/shared/fiber/ReactFiberBeginWork.js
+++ b/src/renderers/shared/fiber/ReactFiberBeginWork.js
@@ -338,8 +338,10 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         return bailoutOnAlreadyFinishedWork(current, workInProgress);
       }
       const element = state.element;
+      const root: FiberRoot = workInProgress.stateNode;
       if (
         (current === null || current.child === null) &&
+        root.hydrate &&
         enterHydrationState(workInProgress)
       ) {
         // If we don't have any current children this might be the first pass.
diff --git a/src/renderers/shared/fiber/ReactFiberHydrationContext.js b/src/renderers/shared/fiber/ReactFiberHydrationContext.js
index c1475017dd7..f6024e39c67 100644
--- a/src/renderers/shared/fiber/ReactFiberHydrationContext.js
+++ b/src/renderers/shared/fiber/ReactFiberHydrationContext.js
@@ -77,9 +77,8 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     didNotMatchHydratedTextInstance,
     didNotHydrateContainerInstance,
     didNotHydrateInstance,
-    // TODO: These are currently unused, see below.
-    // didNotFindHydratableContainerInstance,
-    // didNotFindHydratableContainerTextInstance,
+    didNotFindHydratableContainerInstance,
+    didNotFindHydratableContainerTextInstance,
     didNotFindHydratableInstance,
     didNotFindHydratableTextInstance,
   } = hydration;
@@ -140,25 +139,25 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     fiber.effectTag |= Placement;
     if (__DEV__) {
       switch (returnFiber.tag) {
-        // TODO: Currently we don't warn for insertions into the root because
-        // we always insert into the root in the non-hydrating case. We just
-        // delete the existing content. Reenable this once we have a better
-        // strategy for determining if we're hydrating or not.
-        // case HostRoot: {
-        //   const parentContainer = returnFiber.stateNode.containerInfo;
-        //   switch (fiber.tag) {
-        //     case HostComponent:
-        //       const type = fiber.type;
-        //       const props = fiber.pendingProps;
-        //       didNotFindHydratableContainerInstance(parentContainer, type, props);
-        //       break;
-        //     case HostText:
-        //       const text = fiber.pendingProps;
-        //       didNotFindHydratableContainerTextInstance(parentContainer, text);
-        //       break;
-        //   }
-        //   break;
-        // }
+        case HostRoot: {
+          const parentContainer = returnFiber.stateNode.containerInfo;
+          switch (fiber.tag) {
+            case HostComponent:
+              const type = fiber.type;
+              const props = fiber.pendingProps;
+              didNotFindHydratableContainerInstance(
+                parentContainer,
+                type,
+                props,
+              );
+              break;
+            case HostText:
+              const text = fiber.pendingProps;
+              didNotFindHydratableContainerTextInstance(parentContainer, text);
+              break;
+          }
+          break;
+        }
         case HostComponent: {
           const parentType = returnFiber.type;
           const parentProps = returnFiber.memoizedProps;
diff --git a/src/renderers/shared/fiber/ReactFiberReconciler.js b/src/renderers/shared/fiber/ReactFiberReconciler.js
index e0972a0314a..7f6a7c3017c 100644
--- a/src/renderers/shared/fiber/ReactFiberReconciler.js
+++ b/src/renderers/shared/fiber/ReactFiberReconciler.js
@@ -219,7 +219,7 @@ type HydrationHostConfig<T, P, I, TI, C, CX, PL> = {
 };
 
 export type Reconciler<C, I, TI> = {
-  createContainer(containerInfo: C): OpaqueRoot,
+  createContainer(containerInfo: C, hydrate: boolean): OpaqueRoot,
   updateContainer(
     element: ReactNodeList,
     container: OpaqueRoot,
@@ -335,8 +335,8 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   }
 
   return {
-    createContainer(containerInfo: C): OpaqueRoot {
-      return createFiberRoot(containerInfo);
+    createContainer(containerInfo: C, hydrate: boolean): OpaqueRoot {
+      return createFiberRoot(containerInfo, hydrate);
     },
 
     updateContainer(
diff --git a/src/renderers/shared/fiber/ReactFiberRoot.js b/src/renderers/shared/fiber/ReactFiberRoot.js
index f78205d2bca..419e842e08b 100644
--- a/src/renderers/shared/fiber/ReactFiberRoot.js
+++ b/src/renderers/shared/fiber/ReactFiberRoot.js
@@ -26,9 +26,14 @@ export type FiberRoot = {
   // Top context object, used by renderSubtreeIntoContainer
   context: Object | null,
   pendingContext: Object | null,
+  // Determines if we should attempt to hydrate on the initial mount
+  +hydrate: boolean,
 };
 
-exports.createFiberRoot = function(containerInfo: any): FiberRoot {
+exports.createFiberRoot = function(
+  containerInfo: any,
+  hydrate: boolean,
+): FiberRoot {
   // Cyclic construction. This cheats the type system right now because
   // stateNode is any.
   const uninitializedFiber = createHostRootFiber();
@@ -39,6 +44,7 @@ exports.createFiberRoot = function(containerInfo: any): FiberRoot {
     nextScheduledRoot: null,
     context: null,
     pendingContext: null,
+    hydrate,
   };
   uninitializedFiber.stateNode = root;
   return root;
diff --git a/src/renderers/testing/ReactTestRendererFiberEntry.js b/src/renderers/testing/ReactTestRendererFiberEntry.js
index 1b0253f5125..79144a1449a 100644
--- a/src/renderers/testing/ReactTestRendererFiberEntry.js
+++ b/src/renderers/testing/ReactTestRendererFiberEntry.js
@@ -574,7 +574,7 @@ var ReactTestRendererFiber = {
       createNodeMock,
       tag: 'CONTAINER',
     };
-    var root: FiberRoot | null = TestRenderer.createContainer(container);
+    var root: FiberRoot | null = TestRenderer.createContainer(container, false);
     invariant(root != null, 'something went wrong');
     TestRenderer.updateContainer(element, root, null, null);