This article reviews recent research evidence, largely from systematic reviews, on a number of aspects of childhood obesity: its definition and prevalence; consequences; causes and prevention. The basis of the body mass index (BMI) as a means of defining obesity in children and adolescents is discussed: a high BMI for age constitutes obesity. In recent years the prevalence of obesity has increased dramatically across most of the world, and in the UK obesity prevalence is high and continuing to increase. Despite a widespread perception that obesity in childhood or adolescence is a cosmetic issue, the research evidence shows that it does matter to physical and psychological health and that there are adverse health risks for both the obese child and the adult who was obese as a child. Few interventions aimed at preventing or treating childhood obesity have been successful. There is an urgent need for more research on better strategies that will enable children and adolescents to make long-term changes to their dietary and physical activity behaviour in order to prevent obesity.