There is a growing belief among policymakers and the general public that competitive foods in schools are a significant contributor to the childhood obesity epidemic. Numerous policy initiatives are underway at the local, state and federal level to regulate the availability of competitive foods in schools. However, the existing empirical evidence motivating these efforts is limited and rarely addresses the potential endogeneity of the school food environment. In this paper, the authors estimate the causal effect of competitive food availability on children's body mass index (BMI) and other food- and school-related outcomes using an instrumental variables approach on a national sample of children. They find that competitive food availability generates in-school purchases of junk foods, but contrary to common concerns, there is no significant effect on children's BMI. Nor do they observe significant changes in overall consumption of healthy and unhealthy foods, and in physical activity. Finally, their results find no support for broader effects of junk foods in school on social/behavioral and academic outcomes.