iMac dual monitor setup FAQ: Can I connect an external monitor to my iMac? (Or, how do I use the Apple Mini DisplayPort to connect a second monitor to my iMac or MacBook Pro?)
Yes, you can connect a second monitor (external monitor) to your iMac -- here's how.
iMac dual monitor - iMac Mini DisplayPort
To create a dual monitor iMac, first locate the "Mini DisplayPort" on the back of your iMac. You can see the iMac Mini DisplayPort in this photo: