Obesity is a condition resulting from excess body fat, and is associated with several risk factors for chronic diseases in later life. Hence, the prevention of obesity is a public health priority, with much of the concern focusing on childhood and adolescence. Recently, body mass index, calculated as weight divided by height squared (BMI, kg/m²), has been widely used as a surrogate measure of adiposity for children and adolescents, and international age- and sex-specific cut-off points to define overweight and obesity have been proposed. Various surveys using the international BMI criteria have revealed that the prevalence of child obesity is accelerating throughout the developed world. However, BMI is a measure of excess weight relative to height, rather than excess body fat. The interpretation of BMI among children and adolescents is further complicated by the changes that occur in weight, height, and body composition during growth. There is a now a considerable body of evidence that during the growth stages, weight increase is frequently due to an increase in fat-free mass rather than fat mass. Therefore, application of the BMI norm to define overweight and obesity in children and adolescents should be done with caution. Furthermore, it is necessary to be aware that adopting an international standard for a specific population may introduce error because the relationship between BMI and adiposity is race-specific. For this reason, it is expected that race-specific standards of BMI with additional body composition-related information for children and adolescents will become available in the future to ensure reliable assessment of adiposity levels.