Overweight and obesity represent a rapidly growing threat to the health of populations in an increasing number of countries. Indeed they are now so common that they are replacing more traditional problems such as undernutrition and infectious diseases as the most significant causes of ill-health. Obesity comorbidities include coronary heart disease, hypertension and stroke, certain types of cancer, non-insulin-dependent diabetes mellitus, gallbladder disease, dyslipidaemia, osteoarthritis and gout, and pulmonary diseases, including sleep apnoea. In addition, the obese suffer from social bias, prejudice and discrimination, on the part not only of the general public but also of health professionals, and this may make them reluctant to seek medical assistance. WHO therefore convened a Consultation on obesity to review current epidemiological information, contributing factors and associated consequences, and this report presents its conclusions and recommendations. In particular, the Consultation considered the system for classifying overweight and obesity based on the body mass index, and concluded that a coherent system is now available and should be adopted internationally. The Consultation also concluded that the fundamental causes of the obesity epidemic are sedentary lifestyles and high-fat energy-dense diets, both resulting from the profound changes taking place in society and the behavioural patterns of communities as a consequence of increased urbanization and industrialization and the disappearance of traditional lifestyles. A reduction in fat intake to around 20-25% of energy is necessary to minimize energy imbalance and weight gain in sedentary individuals. While there is strong evidence that certain genes have an influence on body mass and body fat, most do not qualify as necessary genes, i.e. genes that cause obesity whenever two copies of the defective allele are present; it is likely to be many years before the results of genetic research can be applied to the problem. Methods for the treatment of obesity are described, including dietary management, physical activity and exercise, and antiobesity drugs, with gastrointestinal surgery being reserved for extreme cases.