Recognized through the ages, OCD is marked by homogeneous concerns of harm, risk, and danger from heterogeneous and ubiquitous sources of contamination, doubt, aggression, and loss. Obsessions are unwanted, intrusive, repetitive ideas, images, and impulses that patients usually recognize as inappropriate or excessive. Compulsions are urges, often strong, to do something to lessen distress from obsessions. Rituals are repetitive, purposeful behaviors, typically tied to obsessions and intended to diminish obsessions or the discomfort they cause. Thus, contamination obsessions lead to washing and cleaning rituals, doubt produces checking, aggressive obsessions evoke avoidance of the objects of aggression, and worry about loss produces hoarding.