Obsessive-compulsive disorder (OCD) is characterized by recurrent, intrusive thoughts (obsessions) and repetitive behaviors (compulsions), which cause distress and impede functioning. OCD symptoms often have common themes, such as contamination, aggression, symmetry, or morality. This disorder can be quite debilitating, time-consuming, and disruptive if left untreated. As a result, it can significantly impair the functioning of affected individuals. Both children and adults can manifest OCD. In children, daily academic performance, as well as social and family functioning can be affected. OCD also commonly co-occurs with learning disorders, which can further impede children’s ability to reach their true potential at school. This entry discusses the diagnosis, assessment, course, etiology, and [Page 1165]treatment of OCD, with specific attention to these issues in youth.