One of the challenges facing modern psychiatry is to determine to what extent the diagnostic categories clinicians have represent valid constructs. Epidemiologic studies are helpful in this regard when their findings are consistent across various cultural or geographic settings or with those of clinical studies. The cross-national epidemiologic data on OCD reviewed in this article are remarkable for their consistency in rates, age at onset, and comorbidity across diverse countries, a fact which lends additional support to the validity of the diagnosis of OCD. The variability in symptom presentation across national sites suggests that cultural factors may affect the symptom expression; however, why the rates of OCD and other psychiatric disorders are so much lower in Taiwan than in other sites, including another Asian site, is unclear. Epidemiologic studies of adolescents and of adults have shown similar prevalence of OCD and substantial comorbidity with major depression and other anxiety disorders. Studies of adolescent populations indicate that OCD symptoms are fairly common among adolescents but not necessarily predictive of developing the full disorder within 1 year of follow-up. Family studies have suggested an association between OCD and TS and other CMT disorders. Clinical studies have suggested an association between Sydenham's chorea and OCD. These various studies provide a growing body of knowledge regarding the nature of OCD. Together with evidence of the substantial demand on mental health services by those afflicted with OCD, the epidemiologic data make a compelling case for additional efforts to improve the understanding and treatment of this troubling disorder.