Obsessive-compulsive disorder (OCD) is classified under anxiety disorders. OCD is characterized by obsessions that are repetitive, senseless, and intrusive thoughts that generally increase anxiety, and/or compulsions that are repetitive, senseless behaviors that often serve to reduce the anxiety caused by the obsession. Common obsessions include an unrealistic and excessive fear of danger, of contamination, or of committing hurtful or morally unacceptable actions. Common compulsions include repetitive cleaning, checking, ordering, arranging, and hoarding behaviors.

Although these symptoms can become debilitating—truly taking over a person’s life—an individual with OCD always retains some degree of insight into the pathology of their behavior. This differentiates OCD from a delusion, in which the person is convinced of the truth of his or her belief. In a mild case of OCD, a person may need to perform a specific routine when turning off the computer at work every evening, perhaps taking fifteen minutes more than necessary to complete the task. In an extreme case of OCD, someone can take nine hours to finish a shower, washing each body part multiple times in ritualized ways.