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Depends on your iMac model...
as a 21.5" iMac, I think it must have a mini Display Port (some older iMacs had mini DVI instead).

THe Monitor has both DVI (Single Link) and VGA inputs.

You need an adapter to go from the output on your iMac to the cable that you want to use. Apple sells these, but you can find cheaper versions elsewhere (monoPrice maybe?) What you don't need is the Dual-Link DVI Adapter, which costs $99 from Apple. It looks like the Monitor includes the actual cable - the adapter is only about 6" long...

I'd go for the DVI Adapter over VGA - pretty sure they are the same price.
 
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