Obesity in children and adolescents has reached alarming levels--20%-25% of children and adolescents are overweight or obese, and 4.9% of boys and 5.4% of girls are obese. Rates of obesity have increased significantly in Australia from 1985 to 1995, with the prevalence of overweight doubling and obesity trebling. Body mass index (related to reference standards for age and sex) is recommended as a practical measure of overweight and obesity in children, and is used in monitoring individual progress in clinical practice. Obesity in childhood and adolescence may be associated with a range of medical and psychological complications, and can predispose individuals to serious health problems in adult life, including type 2 diabetes, hypertension, dyslipidaemia and non-alcoholic steatohepatitis. Obesity interventions for which there is some evidence include family support, a developmentally appropriate approach, long-term behaviour modification, dietary change, and increased physical activity and decreased sedentary behaviour. Prevention of obesity in children and adolescents requires a range of strategies involving changes in both the microenvironment (eg, housing, neighbourhoods, recreational opportunities) and the macroenvironment (eg, food marketing, transport systems, urban planning).