Studies in a variety of countries have shown increases in the prevalence of overweight among children in recent years. These increases have given rise to concern about children's health and well-being. The terminology used in these studies varies considerably. However, whatever the terminology used, such studies are generally based on weight [expressed as body mass index (BMI), a measure of weight for height, calculated as weight in kilograms divided by the square of height in meters] and not on body fatness per se. There are many different BMI references that can be used to define childhood overweight or obesity. Children are defined as overweight for population surveillance purposes using a variety of BMI cut points. BMI is a screening tool, not a diagnostic tool. Children with a BMI over these cut points do not necessarily have clinical complications or health risks related to overfatness. More in-depth assessment of individual children is required to ascertain health status. The definitions of overweight generally used are working definitions that are valuable for general public health surveillance, screening and similar purposes.