Time machines and the breakdown of unitarity

We present a generic way of thinking about time machines from the view of a faraway observer. In this model the universe consists of three (or more) r egions: One containing the entrance of the time machine, another the exit, and the remaining one(s) the rest of the universe. In the latter we know or dinary quantum mechanics to be valid and thus are able to write down a Hami ltonian describing this generic time machine. We prove that the time evolut ion operator is nonsymmetric. Various interpretations of this irreversibili ty are given.