With the abroad application of instant messaging and explosive growth of users in current years, IM worms have higher outbreak frequency, wider coverage and deeper hurt, which have been a serious threat to network security. In this paper, the concept and research situation of IM (instant messaging)worms, function component, execution mechanism and comparison with other worms are presented, then the network topology and propagation models are discussed, and finally the critical techniques of IM worm prevention are given. Some major problems and research trends in this area are also addressed.