People with Obsessive-Compulsive Disorder (OCD), an anxiety disorder, suffer intensely from recurrent, unwanted thoughts (obsessions) and/or repetitive behaviours (compulsions) that they feel they cannot control. Repetitive behaviours such as hand-washing, counting, checking, or cleaning are often performed with the hope of preventing obsessive thoughts or making them go away. Performing these so-called "rituals," however, provides only temporary relief, and not performing them markedly increases anxiety. Left untreated, obsessions and the need to carry out rituals can take over a person's life. OCD is often a chronic, relapsing illness. The first symptoms of OCD often begin during childhood or adolescence. OCD is equally common in males and females. OCD is sometimes accompanied by depression, eating disorders, substance abuse, or other anxiety disorders. Symptoms of OCD can also coexist and may even be part of a spectrum of other brain disorders, such as Tourette's syndrome. Appropriate diagnosis and treatment of other co-occurring disorders are important to successful treatment of OCD. This new volume offers new research from around the world.