Additional Study Information: Obssesive Compulsive Disorder (OCD) and related disorders characterized by repetitive thoughts and behaviors (e.g., Tourette Syndrome and Eating Disorders) are severe conditions that lead to significant health-related disability. Dysfunction in the brain circuits involved in cognitive control and reward processing are thought to contribute to the symptoms of these disorders. This study will use neuroimaging to examine control and reward circuits in 40 unmedicated adults with OCD and 40 matched healthy control subjects. Participants will also complete interviews, questionnaires, and behavioral tasks of cognitive control and reward processing. OCD patients will then be offered a standard course of Exposure and Ritual Prevention (EX/RP) (17 sessions delivered twice-weekly) before rescanning (along with 20 healthy controls) in order to see how these brain circuits may change with treatment.