diff --git a/README.md b/README.md
index 8bf59aa..bc812fd 100644
--- a/README.md
+++ b/README.md
@@ -29,52 +29,86 @@ function MyComponent() {
 
 ## ResizeObserver
 
-[Resize Observer](https://developers.google.com/web/updates/2016/10/resizeobserver)
-is the API is used to determine if an element is resized. Browser support is pretty good in Chrome, but is still missing support in other major browsers.
+If it is present, this library uses the recent [`ResizeObserver` browser
+API](https://developers.google.com/web/updates/2016/10/resizeobserver) to
+determine if an element's content size has changed.
+
+If a browser does not have the `ResizeObserver` API present, then this library
+falls back to listening on window size changes, which is less efficient and does
+not listen for changes to the component's size due to other factors like content
+changes. If it is not present, you can use pass a `ResizeObserver`
+implementation into the `useComponentSize()` hook (see below).
+
+Browser support is pretty good in Chrome, but is still missing support in other
+major browsers.
 
 > [Can i use ResizeObserver?](https://caniuse.com/#feat=resizeobserver)
 
 ### Polyfill
 
-You can import the
-[polyfill](https://github.com/que-etc/resize-observer-polyfill) directly from here
+You can import [a ResizeObserver
+ponyfill](https://github.com/que-etc/resize-observer-polyfill) with this NPM
+library:
 
 ```sh
 yarn add resize-observer-polyfill
 ```
 
-Then import it in your app:
+Then use it with the `useComponentSize()` hook:
 
 ```js
-import 'resize-observer-polyfill'
+import ResizeObserver from 'resize-observer-polyfill'
+// ...
+useComponentSize(ref, { ResizeObserver });
 ```
 
 If you are using Webpack (or similar) you could use [dynamic
-imports](https://webpack.js.org/api/module-methods/#import-), to load the
-Polyfill only if needed. A basic implementation could look something like this:
+imports](https://webpack.js.org/api/module-methods/#import), to load the
+Polyfill only if needed. A basic implementation in your component could look
+something like this:
 
 ```js
-loadPolyfills()
-  .then(() => /* Render React application now that your Polyfills are ready */)
-
-/**
-* Do feature detection, to figure out which polyfills needs to be imported.
-**/
-function loadPolyfills() {
-  const polyfills = []
-
-  if (!supportsResizeObserver()) {
-    polyfills.push(import('resize-observer-polyfill'))
+// GetResizeObserver.js
+// Ponyfill, not polyfill, since we're not chaging the global
+// `window.ResizeObserver` variable
+let ResizeObserverPonyfill;
+
+export async function getResizeObserverOrPonyfill() {
+  const BuiltinResizeObserver = window.ResizeObserver;
+  if (BuiltinResizeObserver) {
+    return BuiltinResizeObserver;
   }
-
-  return Promise.all(polyfills)
+  ResizeObserverPonyfill = (await import("resize-observer-polyfill")).default;
+  return ResizeObserverPonyfill;
 }
 
-function supportsResizeObserver() {
-  return (
-    'ResizeObserver' in global &&
-    'ResizeObserverEntry' in global &&
-    'contentRect' in ResizeObserverEntry.prototype
-  )
-}
+```
+
+```js
+// Your component
+const [ResizeObserver, setResizeObserver] = useState(
+  window.ResizeObserver);
+
+useEffect(() => {
+  if (ResizeObserver) {
+    return; // don't need to load the ponyfill
+  }
+  let cancelled = false;
+
+  // if imported multiple times, should load from browser cache;
+  // or you can cache this in a variable
+  getResizeObserverOrPonyfill().then(lib => {
+    if (!cancelled) {
+      // must wrap `lib` in a function: `ResizeObserver` is a function
+      // itself, so prevent the state hook from interpreting as a reducer
+      setResizeObserver(() => lib);
+    }
+  });
+
+  return () => {
+    // if unmounted before complete, don't call set state
+    cancelled = true;
+  }
+}, []);
+useComponentSize(ref, { ResizeObserver });
 ```
diff --git a/index.d.ts b/index.d.ts
index 8d655d4..107026c 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -3,4 +3,8 @@ interface ComponentSize {
   height: number
 }
 
-export default function useComponentSize<T = any>(ref: React.RefObject<T>): ComponentSize
+interface ComponentSizeOptions {
+  ResizeObserver?: ResizeObserver;
+}
+
+export default function useComponentSize<T = any>(ref: React.RefObject<T>, opts?: ComponentSizeOptions): ComponentSize
diff --git a/index.js b/index.js
index 5e652a2..9f5f7ca 100644
--- a/index.js
+++ b/index.js
@@ -18,7 +18,12 @@ function getSize(el) {
   }
 }
 
-function useComponentSize(ref) {
+function useComponentSize(ref, opts) {
+  var ResizeObserverConstructor = opts && opts.ResizeObserver
+    ? opts.ResizeObserver
+    : typeof ResizeObserver === 'function'
+      ? ResizeObserver
+      : undefined;
   var _useState = useState(getSize(ref ? ref.current : {}))
   var ComponentSize = _useState[0]
   var setComponentSize = _useState[1]
@@ -40,8 +45,8 @@ function useComponentSize(ref) {
 
       handleResize()
 
-      if (typeof ResizeObserver === 'function') {
-        var resizeObserver = new ResizeObserver(function() {
+      if (ResizeObserverConstructor) {
+        var resizeObserver = new ResizeObserverConstructor(function() {
           handleResize()
         })
         resizeObserver.observe(ref.current)
@@ -58,7 +63,7 @@ function useComponentSize(ref) {
         }
       }
     },
-    [ref.current]
+    [ref.current, ResizeObserverConstructor]
   )
 
   return ComponentSize
diff --git a/index.js.flow b/index.js.flow
index 18dc8a8..e350dfe 100644
--- a/index.js.flow
+++ b/index.js.flow
@@ -3,4 +3,8 @@ interface ComponentSize {
   height: number,
 }
 
-declare export default function useComponentSize<T>(ref: React.Ref<T>): ComponentSize;
\ No newline at end of file
+interface ComponentSizeOptions {
+  ResizeObserver?: ResizeObserver;
+}
+
+declare export default function useComponentSize<T>(ref: React.Ref<T>, opts?: ComponentSizeOptions): ComponentSize;
\ No newline at end of file
diff --git a/package.json b/package.json
index cd856df..082c013 100644
--- a/package.json
+++ b/package.json
@@ -32,7 +32,8 @@
     "raf": "3.4.0",
     "react": "16.8.1",
     "react-dom": "16.8.1",
-    "react-test-renderer": "16.8.1"
+    "react-test-renderer": "16.8.1",
+    "resize-observer-polyfill": "^1.5.1"
   },
   "ava": {
     "require": [