Obesity is a serious problem that affects children from diverse ethnic backgrounds in both industrialised and developing countries. Worldwide, an estimated twenty-two million children <5 years of age were overweight in 2007. In the UK if current trends continue an estimated one-quarter of all children <16 years of age will be obese by 2050. Recent evidence suggests that most obesity is established during the preschool years, and because one in five obese 4 year olds will become obese adults this situation has major implications for public health. The causes of obesity in preschool children are complex and multifactorial. Although 30-50% of the predisposition towards obesity in preschool children can be explained by genetic factors, environmental influences also play a crucial role. The preschool period in particular is a pivotal time during which long-term dietary and physical activity habits are established, with potential lifelong effects on health. However, research in this age-group is limited. Previous studies have aimed to improve diet, increase physical activity and achieve behavioural change. However, few of these studies have been successful and there is an urgent need, therefore, for the development of evidence-based interventions aimed at the prevention of preschool obesity.