  A geometrically thin, optically thick, warped accretion disk with a central
source of luminosity is subject to non-axisymmetric forces due to radiation
pressure; the resulting torque acts to modify the warp. Initially planar
accretion disks are unstable to warping driven by radiation torque, as shown in
a local analysis by Pringle (1996) and a global analysis of the stable and
unstable modes by Maloney, Begelman, & Pringle (1996). In general, the warp
also precesses.
  We discuss the nature of this instability, and its possible implications for
accretion disks in X-ray binaries and active galactic nuclei. Specifically, we
argue that this effect provides a plausible explanation for the misalignment
and precession of the accretion disks in X-ray binaries such as SS 433 and Her
X-1; the same mechanism explains why the maser disk in NGC 4258 is warped.
