This three-paper dissertation contributes to the broader understanding of the complex tobacco use behaviors such as exclusive and dual use of cigarettes and e-cigarettes, the transitions between them and the factors associated, in consideration of the COVID-19 pandemic, cannabis legalization policies as well as concurrent use of cigarettes and e-cigarettes in the initial weeks following initiation of e-cigarettes by adults who smoke cigarettes. The first paper examines the quantitative survey data from a national probability online sample of 1460 US adults, and qualitative data from an online focus group study of 61 adults from Atlanta, GA, US. The study found significant differences in tobacco purchasing behaviors between exclusive and dual users of cigarettes and e-cigarettes as well as identified risk perceptions, prices of tobacco products and access to tobacco product as factors driving the changes in tobacco purchasing behaviors during COVID-19 pandemic. With the rapidly changing cannabis policies in the US, the second paper reports the findings from a systematic review to understand the impact of such policies on the tobacco use of adolescents and young adults. The findings suggest a lower likelihood of cigarette use in states with cannabis legalization as compared to states with no cannabis legalization. The review also highlights the limitations in the existing literature potentially responsible for continued uncertainty in the association. Overall, this paper may inform state, local, and federal policymakers, the relationship between state cannabis legalization and tobacco use among adolescents and young adults in the US and guides future research to develop robust findings. The third paper examines the transitions between cigarette and e-cigarette use behaviors over three months, among adult smokers following their past-month initiation or re-initiation of e-cigarettes. The study found a higher likelihood of continuing the same behavior for exclusive users as compared to dual users. Dual users were more likely to transition towards the exclusive use of the tobacco product they predominantly used than to remain stable. The findings from this paper informs policymakers, researchers ad physicians the importance of early interventions for smoking cessation specifically during the experimentation period of e-cigarette use for adult smokers.