The prevalence and degree of obesity has increased dramatically over the past two decades. Obese children have a high probability of becoming obese adults. For example, an obese child, who remains obese through adolescence, has approximately a 28 to 1 chance of attaining normal weight as an adult. Sedentary behavior, which has its origins in childhood and adolescence, is strongly associated with obesity. Establishing enthusiasm for physical activity and consumption of nutritious, lower fat snacks at an early age may act to promote and preserve interest in these healthy behaviors as children grow older. Because children are more adaptable to changes in lifestyle than adults, and because prevention is more feasible than treatment, we believe that young children are the ideal recipients of our proposed intervention. In this pilot project we propose to develop and conduct a 7 month evaluation of a targeted intervention to potentially decrease the risk and prevalence of childhood obesity by increasing physical activity and the consumption of nutritious lower fat snacks in primary school children who attend YMCA after-school day care programs. Day care providers will receive extensive training and support for the first 3 months which will be withdrawn during the final 4 months of the interventions. We will train YMCA after-school day care staff to deliver 150 minutes per week of moderate intensity physical activity, and to provide reduced fat snacks at 2 randomly selected after-school programs and compare the results with those from 2 schools receiving standard YMCA after-school day care. Our approach satisfies the ACSM/CDC and Healthy People 2010 guidelines for physical activity, provides physical activity to children in a context other than physical education or sport, and targets snacks that are traditionally high in fat and contribute significantly to daily energy intake. Our primary aims are to 1) increase physical activity to a minimum of 150 minutes of moderate physical activity per week and 2) increase consumption of nutritious lower fat snacks in elementary school children attending YMCA after school programs. Our secondary aims are to document the fidelity and extent of implementation of our intervention and to identify factors such as characteristics of the day care staff and students, staff training and support, intensity of the physical activity program, and the physical environment of the day care facility that may be associated with successful intervention implementation. The YMCA is the largest provider of after-school day care in the United States. Thus, if successful, we have tremendous potential for dissemination of the intervention through the national YMCA network.