  This is a brief survey of the current status of Stephen Hawking's
``chronology protection conjecture''. That is: ``Why does nature abhor a time
machine?'' I'll discuss a few examples of spacetimes containing ``time
machines'' (closed causal curves), the sorts of peculiarities that arise, and
the reactions of the physics community. While pointing out other possibilities,
this article concentrates on the possibility of ``chronology protection''. As
Stephen puts it:
  ``It seems that there is a Chronology Protection Agency which prevents the
appearance of closed timelike curves and so makes the universe safe for
historians.''
