This is a detailed description of obsessive compulsive disorder (OCD), which is characterized by recurrent, unwanted and unpleasant thoughts and/or repetitive, ritualistic behaviours, which the person feels driven to perform. The author applies cognitive clinical psychology to this disorder, and explores the theory that OCD may of organic origin. The fields of cognitive and biological research are then linked. The book contains case studies drawn from clinical practice to show how theoretical ideas correspond to treatment.