I will discuss G. Margulis' superrigidity theorem. I will indicate how it arose and how R. Zimmer adapted it to dynamics as part of a larger program initiated by G. Mackey. I will also attempt to explain how this dynamical reformulation found application to the study of Borel reducibility of equivalence relations. (For those who attended the Las Vegas meeting, this will be close to my talk there, but I will try to include some additional detail.)