There is currently a great deal of interest in whether or not circadian rhythms are capable of explaining certain psychiatric phenomena. This short review will briefly explain the concept of endogenous circadian rhythms and then go on to describe some of the attempts to relate this to psychiatry. It is not possible to give a complete discussion in the space available and interested readers are referred to two recent texts on circadian rhythms (Wever 1979, Minors and Waterhouse, 1981). The psychiatric aspects have been reviewed by Sampson and Jenner (1975).