Tobacco use remains a critical public health issue in the United States. Young adults are disproportionately affected by high rates of tobacco use and heavily targeted by tobacco marketing. Social media has become an important source of exposure to tobacco and nicotine product marketing and messaging for young adults. This dissertation examined the prevalence and socio-environmental characteristics associated with young adults’ exposure to and engagement with tobacco-related social media (paper 1); the prospective associations between young adults’ exposure and engagement and tobacco and nicotine product use (paper 2); and young adults’ experiences with tobacco and nicotine product messaging on social media, as well as perceptions of existing e-cigarette social media advertisements (paper 3). Participants were two- and four-year college students from the Marketing and Promotions across Colleges in Texas Study (n=4,384; mean age=20.4, standard deviation=2.32; 64.6% female; 35.5% non-Hispanic white, 30.8% Hispanic, 18.2% Asian, 7.9% African American/black, and 7.6% another race/ethnicity or multi-racial). In paper 1, 30% of students reported past 30-day exposure to cigarette, e-cigarette, hookah, cigar, and/or smokeless tobacco advertising on Facebook, YouTube, Instagram, Twitter, Snapchat, Reddit, and/or Pinterest, and 23% of students reported engagement. Anti-engagement activities such as posting links to anti-tobacco messaging were more prevalent than pro-engagement. Racial/ethnic minorities, dual- and poly-users, higher social media users, students with friends that use tobacco, and students with higher depressive symptoms were significantly more likely to report exposure. Racial/ethnic minorities, two-year college students, poly- and dual-users, higher social media users, and students with higher depressive symptoms were more likely to report pro-engagement. Poly-users, higher social media users, students with friends that use tobacco, and students with higher depressive symptoms were more likely to report anti-engagement. In paper 2, multiple logistic regression analyses revealed exposure to and engagement with tobacco-related social media significantly predicted past 30-day use of e-cigarettes, cigars, and hookah at one-year follow up. Controlling for other social media, exposure to any product advertising via Reddit predicted e-cigarette use. Pinterest exposure predicted cigar use. Snapchat exposure predicted hookah use. Pro-tobacco engagement predicted future use of all products. Anti-tobacco engagement predicted use of cigars and hookah. In paper 3, thematic content analysis of qualitative interviews with a subsample of 30 revealed all participants recalled seeing tobacco or nicotine product messaging on social media, primarily for alternative products like e-cigarettes and hookah. Perceptions of researcher-selected advertisements were generally positive, with students preferring advertisements that did not look like traditional advertisements and conveyed fun and social themes. Findings support a critical need for social media-based federal regulation, countermarketing and health communication campaigns, and intervention focused on tobacco