(If there are any corrections to this information, tell me. There are some facts of which I am not sure.)
He has written well over 400 works of fiction and non-fiction, of which the Foundation series is perhaps the most famous. It is said that he is the only author to have a book in every category of the Dewey Decimal system.
The Foundation Series was begun when he was twenty-one, and ended shortly before his death in April 1992.
(See the Isaac Asimov Home Page for more information.)