Background: Prior to the federal law passed in late 2019, many states passed an increased age of sale law prohibiting youth under age 21 (or Tobacco 21) from purchasing tobacco products and e-cigarettes. Although previous research has documented tobacco retail sales violations, fewer studies have examined age verification and illegal tobacco sales in the context of Tobacco 21 or in repeated purchase attempts in various settings. Methods: In this study conducted between 2019 and 2022, buyers aged 18 to 20 years made repeated unsupervised purchase attempts of cigarettes, cigars, e-cigarettes, tobacco-free nicotine pouches and/or smokeless tobacco in store visits to over 180 tobacco or vape retailers in New Jersey, New York City, and Pitt County, North Carolina. Buyers documented whether they were asked for identification and whether they were able to successfully purchase a tobacco or nicotine product at each visit. Results: The primary outcome will be the percent of retailers that checked buyers’ identification at store visits and the precent of visits that resulted in a successful underage tobacco product purchase. We will aim to compare the results across time periods and study sites while controlling for other factors (including repeated visits). Conclusion: These results will be beneficial to designing compliance check inspections and/or interventions that reduce youth access to tobacco.