Oh, come on. Just go to Richard Belzer’s page on the IMDb:
http://us.imdb.com/Name?Belzer,+Richard
You can see that the character of John Munch was a regular on Homicide and Law & Order: SVU and also appeared in the shows The Beat, Law & Order, and The X Files. Why do people ask these questions when all you have to do is do a thirty-second search on the IMDb to answer them?