Functional psychosis in the puerperal period is a dramatic phenomenon that presents a unique set of diagnostic and therapeutic challenges. Despite its omission from modern classificatory systems such as DSM-III and DSM-III-R, the concept of puerperal psychosis continues to receive support from clinicians on the basis of the apparently distinctive clinical picture that characterizes psychoses occurring at this time. The range of possible nosological models is considered in light of relevant studies of puerperal illness, and the evidence for and against each model is presented. The question of the prognosis of puerperal psychosis is reviewed and guidelines for its estimation in the individual case proposed. The heuristic importance of puerperal psychosis in terms of basic research and preventive psychiatry is also stressed.