There is a gap in understanding how to optimally regulate electronic cigarettes (`e-cigarettes') given unique risks of e-cigarette use (`vaping') and possible spillover effects of vaping regulations on other health behaviors. E-cigarettes are currently used by 16% of high school students and 5% of middle schoolers. Among adults, 13% report having used e-cigarettes at some point. The long-term goal of our proposed research is to assist in optimally regulating e-cigarettes by estimating the impact of e-cigarette regulations on a range of important outcomes. Specifically, the objective of our application is to use rigorous econometric methods (e.g. differences-in-differences models, event studies, changes-in-changes methods, unconditional quantile regressions, factor models, and synthetic control models) and high quality, reproducible data (both proprietary and survey) to causally estimate the effect of e-cigarette regulations on: (i) e-cigarette prices, access, and perceptions; (ii) use and sales of e-cigarettes, traditional cigarettes, cigars, smokeless tobacco products, and dual- and multi-use of these products.; and (iii) sales of nicotine replacement therapy. We will estimate the effect of three regulations currently being implemented by both state and county governments: minimum legal sales ages, taxes, and indoor use bans. Our data sources include Nielsen retail data, Behavioral Risk Factor Surveillance System (BRFSS), Monitoring the Future (MTF), National Health Interview Survey (NHIS), and Youth Risk Behavior Surveillance System (YRBSS). We will perform analyses separately for youth and adults, using variation in e-cigarette regulations through year 2019. In our regression models, we will control for a variety of factors that could otherwise bias our estimates, such as demographic information and regulations targeting tobacco use and use of related substances. A rationale for our proposed research is that smoking is the largest cause of preventable death in the United States, raising the importance of understanding the impact of vaping regulations on tobacco use and nicotine replacement therapy use. Smoking leads to 480,000 deaths and $170 billion in medical expenditures each year. Our project is significant by providing important inputs of costs and benefits of e-cigarettes for policymakers deciding how to regulate optimally e-cigarettes. Our project is also innovative for a variety of reasons, including introducing innovative and powerful research methods (changes-in-changes methods, unconditional quantile regressions, factor models, and synthetic control models) to the e-cigarette literature; carefully examining mechanisms through which e-cigarette regulations may affect the use of e-cigarettes and traditional cigarettes, such as through retail prices and by influencing consumers' perceptions of the absolute and relative risks of these two cigarette types; and developing methods to standardize e-cigarette taxes to estimate a `pass-throug...