This chapter provides an overview of the empirical literature for obsessive-compulsive disorder (OCD) and its treatment. Eating disorders and body dysmorphic disorder are associated with OCD, typically co-occurring in approximately 10% of OCD cases. J. Dollard and N. L. Miller adapted Mowrer's two-stage theory to explain the development and maintenance of OCD. Exposure and response prevention (ERP) is the psychological treatment of choice for mild to moderate OCD, and a combination of medication and ERP is recommended for moderate to severe forms of the disorder. In building a hierarchy and initiating ERP, it is important to begin with an item that produces relatively little anxiety and one in which the patient is confident of successfully completing. When developing a treatment plan for OCD patients, a number of factors need to be considered. Cognitively focused treatments have provided an alternative or adjunct to existing gold standard ERP.