  We investigate the appearance of closed timelike curves in quotients of plane
waves along spacelike isometries. First we formulate a necessary and sufficient
condition for a quotient of a general spacetime to preserve stable causality.
We explicitly show that the plane waves are stably causal; in passing, we
observe that some pp-waves are not even distinguishing. We then consider the
classification of all quotients of the maximally supersymmetric ten-dimensional
plane wave under a spacelike isometry, and show that the quotient will lead to
closed timelike curves iff the isometry involves a translation along the u
direction. The appearance of these closed timelike curves is thus connected to
the special properties of the light cones in plane wave spacetimes. We show
that all other quotients preserve stable causality.
