Recent research has shown that the Supplemental Nutrition Assistance Program (SNAP) effectively reduces food insecurity. Questions remain, however, about the extent to which SNAP affects the quality of adult participants’ diets. These questions have surfaced in the context of the increasing public costs of diet-related illnesses, such as diabetes, high blood cholesterol, and heart disease, and have led to discussions about restricting the use of SNAP benefits to purchase some food items. This report examines Healthy Eating Index (HEI) scores for adults in low-income households that do and do not participate in SNAP. To disentangle the choice of whether to participate in SNAP from diet choices, this model uses a unique data set that matches State-level SNAP policy variables to individual-level data from four waves of the National Health and Nutrition Examination Survey (NHANES). Two important kinds of results emerge: the effect of SNAP on the diet quality of those who choose to enroll, and a total comparison of SNAP participants and nonparticipants after SNAP’s effects are taken into account. On the first, this report shows that SNAP participation results in a large increase in the likelihood of consuming whole fruit and a slightly lower consumption of dark green/orange vegetables. On the second, the report finds that SNAP participants have slightly lower HEI scores (both total and components) than nonparticipants, meaning that they have slightly lower diet quality. They do, however, consume less saturated fat and sodium than nonparticipants.