Back and forward buttons in an iframe

Button within frame:

<input type="button" value="Back" onclick="history.back()">

Button within parent frame:

<input type="button" value="Back" onclick="frame_name.history.back()">

Use the window.history object.

// For the current window
window.history.back();     
window.history.forward();

// For an iframe's window
iframe.contentWindow.history.back(); 
iframe.contentWindow.history.forward();

or

iframe.contentWindow.history.go(-1); // back
iframe.contentWindow.history.go(1);  // forward

https://developer.mozilla.org/en/dom/window.history

Update for 2017: There is no way whatsoever to do this if the origin of the iframe content is different from the origin of the enclosing page - unless you control the content at the remote origin and can have it accept postMessage events. If the origin is the same, the older answers still work.

If this is in a WebView within an application you control, you can also set up a hook native side to control this.