1-in-7 Americans received benefits from the Supplemental Nutrition Assistance Program in 2011, an all-time high. We analyze changes in program enrollment over the past two decades, quantifying the contributions of unemployment and state policy changes. Using instrumental variables to address measurement error, we estimate that a one percent- age point increase in unemployment raises enrollment by 15 percent. Unemployment explains most of the decrease in enrollment in the late 1990s, state policy changes explain more of the increase in enrollment in the early 2000s, and unemployment explains most of the increase in enrollment in the aftermath of the Great Recession.