Obsessive-compulsive disorder (OCD) is prevalent, chronic, and potentially disabling. It is characterized by recurrent, unwanted, and distressing thoughts (obsessions) and repetitive, irresistible, behaviors (compulsions). Individuals with OCD recognize that the obsessions and compulsions are senseless or excessive yet they are unable to stop these behaviors. Some etiologic theories of OCD suggest a biological origin, including hypotheses involving the serotonergic system, the glutamatergic system, the orbital cortex and the basal ganglia, and streptococcal throat infections in children. Standard treatments for OCD include selective serotonin reuptake inhibitors (SSRIs) and cognitive-behavior therapy. Combining SSRIs with other medications has relatively little empirical support; however, the adjunctive use of antipsychotics has been shown to be effective. Neurosurgery, such as deep brain stimulation, has also been shown to be effective in select patients with debilitating and refractory OCD.