[mcphage]

It has a mini display port in the back, and I just run a mini display port cable from my laptop to the iMac. I expected it to be complicated when I was looking into how to do it, but it turned out ridiculously easy, I just needed a $5 cable.

[xanderstrike]

It's worth noting that from 2015 and on, iMacs can no longer be used in target display mode.