Nearly 15 percent of all households and 39 percent of near-poor households were food insecure in 2008. The Supplemental Nutrition Assistance Program (SNAP, formerly called the Food Stamp Program) is the cornerstone of federal food assistance programs and serves as the first line of defense against food-related hardship. Using SIPP data, this paper measures SNAPs effectiveness in reducing food insecurity using an instrumental variables approach to control for selection bias. Our results suggest that SNAP receipt reduces the likelihood of being food insecure by roughly 30 percent and reduces the likelihood of being very food insecure by 20 percent.