I briefly review the theory of tidal instabilities in the accretion disks of close binaries. Superhump‐type disturbances result from the presence of the 3:1 resonance inside the disc, requiring mass ratiosq=M2/M1<qcrit&bartil;0.25–0.33. Tidal instabilities seem to occur in all cataclysmic variables which satisfy this condition. A number of low‐mass x‐ray binaries also exhibit similar phenomena; their presence offers a new way of obtaining information about the component masses of these systems. This may be particularly useful for suspected black‐hole binaries. The period excess associated with superhump phenomena may be rather small in LMXBs with very small mass ratios, and other signatures of tidal instabilities, such as possible vertical disc structure effects, should be looked for. ©1994 American Institute of Physics.