 This study will develop and test a comprehensive intervention involving simple messages that will integrate changes in dietary intake with changes in energy expenditure, while simultaneously modifying structural and environmental factors to promote social support and opportunities for behavioral change. The study will recruit and randomize 48 worksites to a 2-year intervention in which the following will be carried out: build a physical activity intervention combining increased daily physical activity and regular, structured exercise; build a dietary intervention that will promote lower calorie intake; and increase worksite access to both healthy foods and physical activity. The primary aim is to evaluate the effectiveness of the intervention in reducing or maintaining body mass index in a randomized controlled trial of worksites. The study will compare changes in body mass index in intervention versus control worksites using two cross-sectional surveys at baseline and follow-up. Additional assessments using biomarkers and fitness measures will be performed in a subset of employees. The research team has considerable experience with interventions at the worksite level and substantial expertise in obesity prevention. 