In the span of a decade, the research image of obsessive compulsive disorder has changed from that of a little known, poorly understood neurosis to one of the most exciting frontiers in psychiatry. It is now recognized as being a common disorder, mainly associated with intrusive, obsessional thoughts or recurrent strong promptings to perform ritualistic behaviour. The ruminations and compulsions cause marked distress and are so time-consuming as to interfere dramatically with normal daily life. Presented here is authoritative and comprehensive coverage of the most recent and significant advances in the basic research and clinical investigation of obsessive compulsive disorder.