Recommended obesity prevention interventions target multiple levels. Effective advocacy is needed to influence factors at individual, social, environmental, and policy levels. This paper describes the rationale for engaging youth in obesity prevention advocacy efforts targeting environment and policy changes to improve nutrition and physical activity. Advocacy involves education, skill development, and behavior and attitude changes, with the goal of persuading others or taking action. Youth advocacy has been successfully used in substance use prevention, but it is relatively new in obesity prevention. A model is presented to guide intervention and evaluation in youth advocacy for obesity prevention. With youth advocacy as a central construct, the model outlines inputs and outcomes of advocacy at individual, social environment, built environment, and policy levels. The model can be used and refined in youth advocacy evaluation projects. By involving youth in their communities, advocacy can produce ownership, engagement, and future involvement yielding sustainable changes.