In this paper I review the observational evidence for long‐term superorbital periodicities and quasi‐periodic phenomena in x‐ray binaries with time scales of ∼30 days to ∼10 years, and discuss their physical interpretation. Superorbital periods may be directly measured from long‐term x‐ray monitoring, inferred by spectroscopic or ‘‘circumstantial’’ evidence, or (perhaps) deduced from changes in the binary orbital period. Possible causes of these periods are: precision processes within the binary; irradiation of the companion by the compact object and/or stellar activity in the companion star; or the presence in the system of a third body, making the source a hierarchical triple. Each of these processes has its own implications for binary evolution. ©1994 American Institute of Physics.