Warps of disk galaxies are a common phenomenon (as common as spiral structure), yet they are still not fully understood. In this review I will try to summarize their observed properties, including preliminary results of a new HI survey we have carried out with Westerbork, and try to relate these to several proposed mechanisms for explaining warps. I will not attempt a historic review of the theoretical work on this subject; such reviews can be found in Binney (1992), or Kuijken (1998), for example.