Obsessive-compulsive disorder is common, affecting up to 3% of adults and children. Obsessions are anxiogenic intrusive thoughts, images or impulses. Compulsions are anxiolytic thoughts or behaviours which reduce the anxiety induced by obsessions. The relief of anxiety reduction maintains the obsessive-compulsive cycle. Symptoms range from mild to life-threatening, where lack of personal care can have potentially lethal consequences. A high proportion demonstrate suicidal behaviours. Most patients, irrespective of symptom severity, respond to treatment with graded exposure or with serotonin reuptake inhibitors, but some continue with a chronic or chronic relapsing course requiring ongoing psychiatric input and specialist interventions.