In 2003, about 56 percent of those eligible to participate in the Food Stamp Program actually participated. The participation rate varied substantially across States, ranging from a high of 83 percent in Oregon to a low of 43 percent in Massachusetts. Using data for 2003 from the Food Stamp Program Quality Control and Current Population Survey, this study examined factors that help to explain the variation. Results show that different population characteristics across States are a major factor because different types of eligible people tend to participate at different rates. States with a higher share of households headed by elderly people had lower rates, while those with a higher share of households without earnings and headed by nonelderly people had higher participation rates. Yet, substantial variation remained after “standardized” State participation rates were calculated that adjust for these compositional differences. Attempts to further explain these standardized rates by State policies and economic conditions were unsuccessful, perhaps due to the limited sample size and imprecise measures of policies.