Obsessive compulsive disorder (OCD) is characterised by persistent, intrusive, and distressing obsessions (unwanted thoughts, impulses, or images) that may or may not be accompanied by compulsions (repetitive behaviours or mental acts). It is a severe and persistent psychological problem with significant negative effects on the individuals' social, family, and occupational functioning. This book reports on recent research examining phenomenology, neurobiology, psychological models, and computational modelling of obsessive-compulsive disorder. This book offers a general view of the most recent psychological and neurobiological research on OCD to researchers, psychologists, psychiatrists, mental-health professionals and students.