SUMMARY The prevalence of overweight and obesity among children and adolescents has been increasing dramatically with low-income and some racial/ethnic minority populations being at highest risk.  Rising obesity rates and socioeconomic disparities in obesity stem from social and environmental factors that need to be understood and modified for effective prevention.  In this project, we propose to follow a cohort of 20,000 children through elementary school, a period when demographic disparities in obesity manifest themselves, to study their physical development, health behaviors, and the relationship of these outcomes and behaviors to the food and physical activity environments in their residential and school neighborhoods.  Our specific aims are as follows: 1.  Describe how the food and physical activity environments around residences and schools differ for children from different socioeconomic and racial/ethnic groups.  2.  Describe how the food and physical activity environments inside elementary schools differ for children from different socioeconomic and racial/ethnic groups.  3.  Analyze the relationship between food and physical activity environments and health behaviors, by socioeconomic and racial/ethnic groups.  4.  Analyze the contribution of food and physical activity environments in the development of disparities in body mass index (BMI) in children from kindergarten through 5th (and for some analyses 8th) grade.  RELEVANCE School and residential environments are likely to shape children's health behaviors, but their contribution to excess weight gain are unknown.  This project tries to identify environmental factors of excess weight gain during elementary school years, which includes the age where sociodemographic disparities in overweight begin to manifest themselves.  This is a necessary task before considering whether proposed interventions and policy changes are promising or likely to lead down a blind alley.