The development of behaviour therapy for OCD and its evolution into cognitive behaviour therapy is described, highlighting the importance of a crucial series of experiments conducted by Rachman and colleagues in the mid-1970s. More recently, developments in cognitive theory suggest that the key to understanding obsessional problems lies in the way in which intrusive thoughts, images, impulses and doubts are interpreted. The important negative interpretations usually concern the idea that the person's action (or choice not to act) can result in harm to oneself or others. This responsibility interpretation has several consequences (such as motivating neutralising behaviour and other counter-productive strategies, increasing selective attention, increased negative mood); these serve to maintain the negative beliefs and therefore the obsessive-compulsive problem. Both general and specific aspects of cognitive-behavioural treatment are described. A number of treatment strategies which are specific to obsessional problems are described in clinical detail.