More than 22 million children under five are now obese or overweight. Globally, an estimated 10% of school-aged children, between 5 and 17 years old, are overweight or obese, and in some countries this figure is as high as 35%. The situation is getting worse. Although recognised clinically for some time as an important condition that increases risk of ill-health in affected individuals, it is only recently, that obesity has been recognised as a population-wide problem that requires preventive action. Obesity is a major contributor to diseases and disability, the associated health costs are enormous, obesity has already reached epidemic proportions in many countries, and incidence is continuing to increase in children and adults. Disturbingly the epidemic is not confined to developed countries, with many developing countries and those in transition affected. While recognised as a major population health problem, our understanding of the causes of the epidemic is poor, there has been relatively little population-based research that has focused on the prevention of unhealthy weight gain, and as a consequence knowledge regarding how and where best to intervene is limited.This book draws together the existing literature and expertised with a view to helping set the agenda for public health action. The book is divided into three sections. Part 1 provides an overview of the context of the problem. It examines the epidemiology of obesity, the role of behavioural factors, socio-cultural factors and environmental factors in the obesity epidemic. Part 2 reviews interventions across a range of key settings and in different population groups - drawing on existing research that has aimed to increase physical activity, promote healthy eating and prevent obesity at a population level. Given how little research there is that has specifically examined the effectiveness of interventions aimed at preventing obesity per se, Part 3 explores potential opportunities to prevent obesity.