Obesity, defined as a body mass index (BMI) of 30 kg/m2 or more, is common in many parts of the world, especially in the established market economies, the former socialist economies of Europe, Latin America, the Caribbean and the Middle Eastern Crescent. As many as 250 million people worldwide may be obese (7% of the adult population) and two to three times as many may be considered overweight. The prevalence of obesity seems to be increasing in most parts of the world, even where it used to be rare. Increased fatness, measured by a high BMI, a large waist circumference or a high waist/hip circumference ratio, is associated with many chronic diseases as well as with poor physical functioning. Assessments of the prevalence of obesity, and trends in this prevalence over time, are more difficult in children than adults, due to the lack of international criteria for classifying individuals as overweight or obese. The World Health Organization has now recommended the use of BMI-for-age percentiles, but the reference curves are still under development. France. The Netherlands, the UK and the USA are among the countries that have reported recent increases in the prevalence of obesity in children and adolescents. Although there are no accurate estimates of the components of energy balance and their changes over time, the available evidence suggests that the trends in obesity rates are related more to a reduction in energy expenditure than to an increase in caloric intake. Prevention of obesity through the promotion of a healthy lifestyle is among the important challenges for the new millennium, and should start in childhood.