Behaviour is an important factor in both the aetiology and treatment of obesity. Successful long-term weight management ultimately depends on the ability of patients to change their behaviour patterns, particularly with regard to diet and exercise. Many patients find such changes extremely difficult to initiate and maintain. The failure to properly address behavioural modification is part of the reason why so many weight loss programmes cannot produce long-term weight loss, even though initial weight loss is often achieved with relative ease. Four examples of lifestyle changes are discussed in this paper: exercise; relapse prevention; the concept of a reasonable weight; and attribution theory. Behavioural therapy should be an integral part of a multifaceted approach for managing obesity and associated conditions, such as type II (non-insulin-dependent) diabetes.