Obsessive-compulsive disorder (OCD) is a formidable public health problem, rated by the World Health Organization as one of the top 20 most disabling diseases.The shame and secrecy associated with OCD, as well as the lack of recognition of its characteristic symptoms, particularly by primary care physicians, hinders appropriate diagnosis and treatment. Most adults with OCD report a late adolescent or early adulthood onset, yet OCD can occur throughout the life span. Pre-school children have been known to develop the disorder, as have elderly persons. OCD can cause considerable distress and disability; for example, children may fall behind in their education, and adults can become housebound. Comorbid psychiatric disorders have become an area of intense interest. Comorbid depression for example, is frequently the symptom that leads the individual to seek treatment. Patients with schizophrenia frequently develop obsessions and compulsions, and these symptoms are associated with a poorer outcome. The role of second-generation antipsychotics merits further study in the treatment of this subset of patients. An important step toward delineating specific subgroups within the OCD-schizophrenia axis may be the use of endophenotypic markers. Unfortunately, many patients with OCD will have a less than optimal treatment response. Poor treatment response is associated with serious social disability, but there are few clues to guide clinicians in predicting treatment response in patients with OCD, in terms of specific demographic variables, clinical symptoms, or the presence of comorbid psychiatric disorders.