Obsessive compulsive disorder (OCD) involves obsessions and compulsions that cause impairment and distress, and which interfere with children’s developmental adaptation and daily functioning. Furthermore, OCD often disrupts peer and family relationships and school performance and may compromise physical health. Once considered rare, recent epidemiologic studies report prevalence rates ranging from 1% in prepubertal children to 4% in adolescents. In addition, children with OCD are at high risk for comorbid psychopathology. Recent theoretic formulations for OCD encourage the integration of both psychological and biological perspectives. To fully understand the cause and course of OCD, integrated assessment of psychological and biological vulnerability as well as prospective longitudinal studies are needed.