Obesity is one of the most important predictors and causes of such lifestyle diseases as metabolic syndrome, type 2 diabetes, and cardiovascular disease. It is thus imperative to follow the trend longitudinally in the population. Measurement of obesity in children and adolescents is difficult; many studies use the body mass index as a measure, regardless of the fact that it is inappropriate. Skinfolds or the ponderal index should be used, instead. Recent Danish studies suggest that the incidence of childhood and adolescent obesity is increasing and that those who are overweight are even more so today than earlier. Obesity is caused by a number of factors, genetic, social, environmental, and lifestyle, all of which play an important role. One of the main causes of the increase in childhood obesity in Denmark today is the lower level of physical activity than formerly. The prospects for the future are an increase in obesity and incidence of lifestyle diseases with a poorer quality of life and a shorter life time expectancy.