This seven-year project is conducting a controlled evaluation of the impact of changes in the food and physical activity (PA) environments on childhood obesity and related behaviors in four New Jersey cities. These cities have been targeted for interventions by major initiatives, affording the opportunity to conduct a natural experiment. The study aims are to advance our understanding of the relationship between elements of the environment and childhood obesity and related behaviors, assess the impact of specific environmental interventions, and demonstrate an innovative methodology for controlled evaluation of community interventions. The research design relies upon a prospective, longitudinal study of a randomly selected panel of 1,200 children in these cities. The research team collected comprehensive baseline data on obesity-related behaviors and body mass index in 2009-2010 in a study funded by the Robert Wood Johnson Foundation. The data also include documentation of the food and physical activity environments to which these children were exposed (geo-coded for analysis). Comprehensive data collection on children is being repeated in four years and relevant changes in the environment are being monitored over the same period. Changes may include presence of new opportunities for healthy eating or physical activity (e.g., new supermarkets, playgrounds); significant enhancement of existing ones (e.g., addition of healthy foods to convenience stores, expanded physical education in schools), as well as non-intervention related change (e.g., retail outlets that close). Individual-level exposure will be measured by proximity of the environmental change to each child’s residence (using geo-spatial methods). The analysis evaluates the impact of exposure to these changes on a comprehensive set of outcomes including behaviors relevant to food consumption and PA as well as weight status. The research design facilitates an exceptional degree of control in isolating the effects of particular intervention strategies and promises to make a significant contribution to enhancing prevention efforts.