Obsessive-compulsive disorder is one of the more common serious mental illnesses. The shame and secrecy associated with it, as well as lack of recognition of its characteristic symptoms, can lead to delay in diagnosis and treatment. Effective psychological and drug treatments are available for the distressing, time consuming, repetitive thoughts and rituals and the associated functional impairment. This article reviews the presentation and assessment of obsessive-compulsive disorder and discusses the current best treatment options, as well as directions for the future. We searched for the term “obsessive compulsive disorder” in electronic databases and referred to published systematic reviews, including the recently published guideline from the National Institute for Health and Clinical Excellence (NICE). Obsessive-compulsive disorder occurs throughout the life span, and children as young as 6 or 7 present with the characteristic impairing symptoms (box 1). At the other end of the age range, patients may present for the first time in old age. Most adults with the disorder report onset in childhood or adolescence. The condition can result in considerable disability; for example, children may drop out of education and adults can become housebound. The World Health Organization rates obsessive-compulsive disorder as one of the top 20 most disabling diseases. If untreated, it generally persists,1 yet effective, evidence based psychological and drug treatments are available. Recent epidemiological studies report prevalence rates of 0.8% in adults and 0.25% in 5-15 year old children,2 3 although earlier studies suggested rates as high as 1-3% in adults and 1-2% in children and adolescents. People of all ages with obsessive-compulsive disorder understand the senseless nature of their repetitive, unwanted behaviours and intrusive, recurrent thoughts. This may lead to shame, reluctance to seek help, and poor recognition by health professionals. People with the disorder have long delays in accessing effective treatments—17 years …