PROJECT SUMMARY/ABSTRACT The nation’s first healthy checkout policy was implemented in Berkeley, CA in 2021 and will be enforced in 2022, presenting an opportunity to understand policy effects on diet quality. This policy prohibits high-sugar and high-sodium products from checkouts, an area known for impulse purchasing. By potentially lowering consumption of sugary beverages, sweets, and salty snacks—the most common items at checkout—this policy could reduce population risk of obesity and types 2 diabetes. Voluntary checkout standards adopted in other countries have successfully decreased purchases of unhealthy foods and beverages and increased purchases of healthy ones, indicating that a mandatory healthy checkout policy could meaningfully improve diet quality. However, because Berkeley’s policy is the first of its kind, there are no studies on the degree to which a healthy checkout policy changes store food environments and the healthfulness of food and beverage purchases—an objective proxy for population diet quality. This research will evaluate the long-term impact of the nation’s first healthy checkout policy on the healthfulness of store food environments and purchases. By leveraging a natural experiment, these outcomes will be compared between Berkeley stores and stores in three comparison cities using synthetic control and difference-in-differences methods. The first aim is to assess the impact of the policy on store environments at checkout and elsewhere in the store, including the prevalence of unhealthy and healthy products and their contents of added sugar, calorie, and sodium in all 26 intervention stores and a random sample of 81 comparison stores. The second aim is to assess policy impact on purchases of (a) small sizes of snack foods and beverages commonly sold at checkout and (b) all sizes of these products (which accounts for substitution) using store- and product- level sales data from 16 intervention and 172 comparison stores. The third aim is to identify implementation factors that influence policy effectiveness, such as policy support, costs, reach, and stakeholder reactions using interviews with city staff, policymakers, and retailers and surveys of Berkeley residents. This evaluation leverages the baseline and 1- year post-baseline store environment data collected by the research team using a novel photo-based tool. The proposed research is expected to provide the first evidence on the effectiveness of a mandatory healthy checkout policy for improving food environments and store sales and the factors that facilitate or pose barriers to implementation, which can inform policy decisions in other jurisdictions.