The search for subtypes of obsessive-compulsive disorder (OCD) has led to increased appreciation of the importance of distinguishing early (prepubertal) versus later onset, and of tic-related versus non-tic related subtypes, as well as postinfectious forms of the disorder. How these apparent typologies relate to each other remains to be elucidated. Careful longitudinal clinical descriptive studies, as well as the ongoing application of genetic, neuroimaging, and immunologic techniques, promise to advance our understanding of how genotype and environmental factors interact to produce the diverse clinical forms of OCD and to point the way to more effective treatment.