Obsessive-compulsive disorder (OCD), classified as an anxiety disorder in the DSM-IV, now is part of a separate category of Obsessive-Compulsive Disorder and Related Disorders in DSM-5. In OCD, the irrational idea or impulse repeatedly and unwantedly intrudes into awareness. Obsessions (recurring distressing thoughts, such as fears of exposure to germs) and compulsions (repetitive actions such as washing one’s hands many times or cognitions such as counting rituals) are usually recognized by the individual as unwanted or unwarranted and are resisted, but anxiety often is alleviated only by ritualistic performance of the compulsion or by deliberate contemplation of the intruding idea or emotion. Some patients with OCD only experience obsessions, while some experience both obsessions and compulsions. Many patients do not volunteer the symptoms and must be asked about them. There is an overlapping of OCD with some features in other disorders (“OCD spectrum”), including tics, trichotillomania (hair pulling), excoriation disorder (skin picking), hoarding, and body dysmorphic disorder. The incidence of OCD in the general population is 2–3% and there is a high comorbidity with major depression: major depression will develop in two-thirds of OCD patients during their lifetime. Male-to-female ratios are similar, with the highest rates occurring in the young, divorced, separated, and unemployed (all high-stress categories). Neurologic abnormalities of fine motor coordination and involuntary movements are common. Under extreme stress, these patients sometimes exhibit paranoid and delusional behaviors, often associated with depression, and can mimic schizophrenia.