Abstract Youth current e-cigarette use rates have dramatically increased in recent years, reaching 19.8% in 2020. State and municipal governments have passed flavor bans, e-cigarette taxes, and Tobacco 21 laws in part to respond to these high rates of youth e-cigarette use. The long-term goal of our research agenda is to provide information that will allow for optimal regulation of e-cigarettes by identifying intended and unintended effects of e-cigarette policies. Specifically, the objective of our current renewal application is to use quasi-experimental methods (e.g., difference-in-differences) to evaluate contemporary tobacco policies of flavor bans, Tobacco 21 laws, and new e-cigarette tax schema. We will use high quality, reproducible data from the Truth Longitudinal Cohort Study, Population Assessment of Tobacco and Health, Monitoring the Future, and Youth Risk Behavior Surveillance System to estimate: (i) the effect of flavor bans with varying levels of strength on youth e-cigarette use, combustible tobacco product use, and substitution towards the remaining legally available flavored tobacco products; (ii) the effect of Tobacco 21 laws on youth e-cigarette use, combustible tobacco product use (e.g., cigarettes, cigars, hookahs), and source of e-cigarettes; and (iii) the effect of e-cigarette taxes, including new e-cigarette tax schema of sales taxes and two-tier taxes, on youth e-cigarette use and combustible tobacco product use. Our project is significant by carefully examining high rates of youth e-cigarette and alternative combustible tobacco products in the United States; identifying policies to reduce tobacco-related disease and death (the leading cause of preventable death in the United States); by contributing externally- valid estimates of the effect of flavor bans from as many as 330 state and municipality flavor restrictions currently on a wide variety of both flavored and unflavored tobacco product outcomes; by carefully considering the strength of flavor bans including products covered, flavors covered, and retailer exemptions; by considering questions on tax saliency for new types of e-cigarette taxes; and by being among the first to study the effect of Tobacco 21 laws on youth e-cigarette use and where youth obtain e-cigarettes. Our project is innovative by using cutting-edge quasi-experimental methods designed to mimic a randomized control trial including newer methods designed for the presence of heterogenous treatment effects; being the first to use Truth Longitudinal Cohort study data and Population Assessment of Tobacco and Health data for quasi-experimental research; by providing a database of standardized e-cigarette taxes; by providing a workshop on heterogenous treatment effect methods; and by collaborating across economics departments, a leading tobacco control non- governmental organization (Truth Initiative), and schools of policy, public health, and medicine. Truth Initiative will lead our dissemination efforts ...