Obsessive-compulsive disorder is a frequent, chronic, costly, and disabling disorder that presents in several medical settings, but is under-recognised and undertreated. For many years, obsessive-compulsive neurosis was seen as a disorder that provided an important window on the workings of the unconscious mind. Today, obsessive-compulsive disorder is viewed as a good example of a neuropsychiatric disorder, mediated by pathology in specific neuronal circuits, and responsive to specific pharmacotherapeutic and psychotherapeutic interventions. In the future we can expect more precise delineation of the origins of this disorder, with integration of data from neuroanatomical, neurochemical, neuroethological, neurogenetic, and neuroimmunological research.