Despite the observation that obsessive-compulsive disorder (OCD) frequently has its onset during childhood or adolescence, most studies have examined OCD in adults. Because pediatric OCD patients are near illness onset with limited or no medication exposure, this population represents a unique window through which to view the neurobiology of OCD. In this article, the authors summarize data from existing studies of pediatric OCD and attempt to interpret the data within the context of a neurobiologic discourse based largely on research in adults. The authors review current neuroanatomic models of OCD and examine neuroimaging and neuropsychologic studies designed to test those models. In addition, the authors examine irregularities of neurotransmitter systems though to be involved in OCD.