The available literature indicates that OCD affecting children and adolescents is highly prevalent. Pediatric-onset OCD seems to share important similarities with the adult disorder but also shows important differences. For example, the clinical phenotype of OCD is remarkably consistent at all ages with some allowances for developmental expression. Pediatric patients frequently demonstrate poor insight into the nature of their obsessions, which in association with their limited verbal expression may make the diagnosis more difficult. Obsessions involving fear of harm and separation, compulsions without obsessions, and rituals involving family members are more common in younger patients. Treatment response,including serotonergic specificity and the need for robust dosing, is another feature shared by early- and adult-onset OCD. Imporfant differences across the life span can also be identified. Perhaps the clearest difference pertains to age of onset. Age-at-onset data have shown a bimodal distribution of age of onset of OCD, with one peak in preadolescent childhood and another peak in adulthood. Another distinction between child and adult OCD is gender representation. Whereas adult studies report equal gender representation or a slight female preponderance, pediatric clinical samples are clearly male predominant. Patterns of psychiatric comorbidity in pediatric OCD show high rates of tic and mood and anxiety disorders, similar to the patterns in adults, but also show a distinct association with disruptive behavior disorders (ADHD and oppositional defiant disorder) and other specific and pervasive developmental disorders. Family studies indicate that the disorder is highly familial and that a childhood onset of the disorder seems to be associated with a markedly increased risk for familial transmission of OCD, tic disorders, and ADHD. Both scientifically and clinically, the recognition of developmentally specific OCD phenotypes may be valuable. For example, research efforts aimed at identifying OCD-associated genes are likely to be more successful if developmentally homogeneous samples are studied instead of combining data from children, adolescents, and adults, as has been common in OCD studies. Clinical management is also informed by an appreciation of the unique cor-relates of OCD affecting youth, especially comorbidity with chronic tic dis-orders and ADHD and their impact on treatment. The so-called "spectrum disorders" related to OCD are less prominent in children and adolescents than in adults. Although sharing some features with typical OCD, these symptoms are less clearly ego-dystonic and less anxiety producing, frequently provide a measure of gratification, and are less responsive in general to SSRIs. Often cognitive antecedents to these behaviors are less well developed than in more typical OCD, and behavioral interventions are the mainstay of treatment but with more variable success.