Major revisions to include a more extensive introduction and an example of bit commitment. Overlap with independent work by Mayers acknowledged. More recent works by Mayers, by Lo and Chau and by Lo are also noted. Accepted for publication in Phys. Rev. Lett

We show that all proposed quantum bit commitment schemes are insecure because the sender, Alice, can almost always cheat successfully by using an Einstein-Podolsky-Rosen type of attack and delaying her measurement until she opens her commitment.