The literature on the epidemiology and genetics of obsessive compulsive disorder (OCD) is reviewed. The application of recently developed diagnostic criteria, psychometric testing, and biologic markers to epidemiologic studies of OCD should provide important new data for increasing our understanding of the pathogenesis of the disorder. Identification of diagnostic subgroups will be important in reliably evaluating treatment response to current and new therapeutic agents. Directions for potential research in these areas are discussed.