A.I. Avoiding Obstacles?

I've tried researching how to do this with no results. My problem is that in the top-down game i'm developing, enemies just overlap each other when pursuing the main character - you. I have no idea where to start on programming A.I. to go around one another (some travel at different rates). I do, however, know how to prevent them from overlapping, so i'm good on that. Could someone point me in the right direction or give me some knowledge on this matter, because i have none.

Cheers, Holsey.