Policymakers, advocates, and the administrators of the Supplemental Nutrition Assistance Program (SNAP) have long hypothesized that SNAP reduces food insecurity, which is a measure of whether a household experiences food access limitations due to lack of money or other resources. Estimating the effect of SNAP on food insecurity using household survey data has been challenging, however, because households that participate in SNAP can differ in systematic but unobserved ways from eligible households that do not. In this study, the association between SNAP participation and household food security is investigated using data from the largest national survey of the food security of SNAP participants to date. The survey was based on a quasi-experimental design intended to minimize selection bias by comparing information collected from SNAP households within days of entering the program to information obtained after about six months of participation to control for factors unrelated to SNAP. [Results of the research awaiting clearance, expected by summer 2013.]