Obsessive-compulsive disorder (OCD) is a common psychiatric illness that occurs across the entire life span. Although most research on OCD pertains to adults, studies of the disorder in children and adolescents have burgeoned over the last decade. A review of this literature suggests that OCD has a bimodal incidence pattern, with one peak of onset at approximately 10 years of age and another during adulthood, and that the juvenile and adult forms are equally prevalent. Important similarities and differences between the juvenile- and adult-onset forms of OCD can be seen: both show the same clinical phenotype, diagnostic nosology, and dose responsivity, but the early-onset disorder differs in being clearly male preponderant, more highly familial, and associated with a distinct pattern of comorbid psychopathology, including disruptive behavior and specific developmental disorders. These findings have implications both for clinical management and for future research, which could consider age of onset as an important factor in studies of all OCD patients.