Electronic cigarettes, or e-cigarettes, with appealing flavors are extremely attractive to youth and young adults, and an increase in the prevalence of flavored e-cigarette use among these groups has been observed in recent years. Much remains to be learned about the personal characteristics and individual perceptions of flavored e-cigarette use among young adults, as well as the potential influence of existing flavored e-cigarette sales restrictions on preventing youth use of e-cigarettes. Based on the Social Ecological Model, this study investigated the intrapersonal, community, and public policy factors that play a role in flavored e-cigarette use among youth and young adults in the U.S. This dissertation (1) analyzed secondary data of 12,383 U.S. young adults using the wave 1 and 2 surveys (2013–2014 and 2014–2015) of the Population Assessment of Tobacco and Health (PATH) Study; (2) analyzed in-depth interview data collected from 25 young adult cigarette smokers; and (3) examined the content of 121 local flavored e-cigarette sales restrictions. In Study 1, the results show that younger age, female gender, education attainment of high school and above, non-cigarette smoking, and diminished harm perception about e-cigarettes were the prospective predictors of non-tobacco and non-menthol flavored e-cigarette use among young adults. In Study 2, qualitative findings suggest that many young adult cigarette smokers held positive attitudes and beliefs about the role of e-cigarette flavors in smoking reduction. In Study 3, content coding results showed that among all the localities with flavored e-cigarette sales restrictions, 117 (96.7%) applied the restriction to the entire jurisdiction, 11 (9.1%) restricted the sale of menthol flavors, and 16 (13.2%) restricted the sale of flavored e-cigarettes in retail tobacco stores. Compared to the localities that enacted lax restrictions, those with strict or moderate restrictions were more likely to have low adult cigarette smoking prevalence. Findings from this study can help develop interventions and campaign messages to prevent and reduce e-cigarette use among youth and young adults. This study can also be used to inform public health practitioners regarding the strategies to strengthen and expand flavored e-cigarette sales restrictions to curb e-cigarette use among younger generations.