In this review diagnosis, epidemiology, etiology, assessment and treatments of obsessive compulsive disorder (OCD) are discussed. OCD is diagnosed on the basis of a psychiatric interview. OCD can be reliably diagnosed with ADIS, a structured interview for anxiety disorders. The life-time prevalence rate of this chronic disorder is 2.4%. Research on etiological factors has yielded conflicting findings, but treatment of OCD can be successful with serotonergic antidepressants, behavior and cognitive therapy. The treatment effect of cognitive-behavioral therapy is larger than of antidepressants. The combination of antidepressants with behavior therapy treatment does not enlarge the efficacy of behavior therapy alone.