Obesity is a complex phenotype caused by the interaction of environmental and genetic factors. In children and adolescents, the 90th and 97th body mass index (BMI) centiles are used to define overweight and obesity. Obesity rates have increased in many countries over the past 30 years. Changes in eating behaviour and reduced levels of physical activity have been cited as factors underlying this obesity epidemic. The past decade has witnessed the identification of rare monogenic forms of obesity and in recent times, advances in the detection of the first polygenes have been made. Several guidelines have been proposed for the treatment of childhood and adolescent obesity. However, the empirical evidence to support any particular treatment option is scant. Neither the methods nor the results of successful treatment studies can be generalized to the obese population. Somatic and psychological side effects of treatment programs need to be considered. Pharmacological and surgical treatments are only infrequently used in adolescents. Whereas single prevention strategies have yielded positive results, it is becoming increasingly clear that approaches which focus on individual behaviour and the family can have only a small impact. Structural prevention strategies are difficult to impose and to evaluate.