Obsessive-compulsive disorder (OCD) is a psychiatric condition characterized by recurrent obsessions or compulsions that cause significant impairment or distress. Although OCD was once perceived to be relatively rare in childhood, current estimates suggest that as many as half of all adult OCD cases may have their onset in childhood or adolescence. In general, there appears to be a great deal of continuity between the clinical presentation of OCD in children and that in adults. Yet, numerous differences have also been found between child and adult OCD, including differences in sex distribution, patterns of comorbidity, and degree of familial loading. These differences raise the issue of whether obsessive-compulsive symptoms that have their onset in childhood, but perhaps persist into adult life, are meaningfully different from those that emerge de novo in adulthood. In this article, current research on child- and adult-onset OCD is critically reviewed. It is proposed that child-onset OCD represents a phenomenologically and etiologically distinct subtype of OCD, bearing a close genetic relationship to tic-disorders and possibly sharing a common or similar pathogenesis. Clinical implications of the child- versus adult-onset OCD distinction are discussed.