In the UK, the National Institute for Health and Clinical Excellence's guidelines on obsessive-compulsive disorder (OCD) recommend cognitive-behavioural therapy, including exposure and response prevention, as an effective treatment for the disorder. This article introduces a cognitive-behavioural model of the maintenance of symptoms in OCD. It discusses the process of engagement and how to develop a formulation to guide the strategies for overcoming the disorder.David Veale is an honorary senior lecturer at the Institute of Psychiatry, King's College London and a consultant psychiatrist in cognitive-behavioural therapy at the South London and Maudsley Trust