Screen Brightness

I currently have my macbook plugged into an external monitor. When I go in to preferences I cant seem to find the slider to change the brightness of the display.

Can someone tell me where to find it?
Is this option only available when using the built in screen on the macbook?

You should see some buttons on the monitor to change settings like brightness, contrast, etc. You might read the manual or find one online.

Yep. I don't think you will get controls for brightness of an external monitor. It should be on the monitor itself. Even the Apple Cinema displays have brightness controls on the monitor...