The prevalence of childhood obesity is increasing and there are a number of theoretical reasons as to why intervention may be more effective in childhood. There are certain risk times for the development of obesity in childhood, which provide a basis for targeted intervention. In addition, tracking data supports the persistence of obesity, at least in later childhood, as well as cardiovascular risk factors. Physical activity is the discretionary component of energy expenditure and there is evidence that falling levels of physical activity are contributing to the obesity epidemic. Physical activity in children is related to developmental stage, is reduced with increasing age and is influenced by parental physical activity. While there is debate about the immediate health benefits of physical activity to children, there are data to support that lower physical activity levels and sedentary behaviours are associated with a higher prevalence of obesity in children. Physical activity is an accepted strategy in the treatment of established obesity (tertiary prevention). The role of physical activity in the prevention of obesity (primary and secondary prevention) is less clear. However a number of recent school-based interventions directed at either increasing physical activity and/or decreasing sedentary behaviours, have shown encouraging results. On balance, increasing physical activity in children is an attractive and non-restrictive approach to obesity prevention. To adopt this approach requires the support and involvement of many community sectors other than health.