Young adults smoke cigarettes at higher rates than any other age group; understanding the risk factors for smoking in young adulthood is fundamental to informing intervention. This dissertation, in three studies, examines the role of alcohol use, social facilitation expectancies, and interpersonal influences on smoking among college-attending young adults. Samples were comprised of young adults aged 18-24 who had smoked at least one cigarette in the last month. For study 1, latent transition analysis (LTA) was used to identify profiles of alcohol and cigarette co-use at three time points and estimate the probability of movement between groups over time. A three-profile solution emerged at each time with profiles representing varying levels of alcohol and tobacco co-use. The LTA probabilities highlighted instability in use. In study 2, the psychometric properties of a new measure of social facilitation expectancies for smoking (SFE) were evaluated using cross-sectional survey data. A nine-item, one-factor scale was confirmed. Higher SFE scores were associated with greater smoking experience and with greater endorsement of other smoking related beliefs. For study 3, latent class growth analysis was used to extract distinct smoking trajectories and examine the effects of demographic, alcohol, and interpersonal factors on trajectory membership. Five smoking trajectories were identified and labeled based on smoking frequency and whether the rate of change indicated stable, decreasing, or increasing use over time. Sex, average number of cigarettes smoked per day, nicotine dependence, and percent of friends who smoke differed between groups, whereas alcohol use did not. Young adult smoking is a temporally unstable behavior, particularly for those using at low levels, and often occurs in the context of alcohol use. Surprisingly, even though these behaviors frequently co-occur, our findings suggest alcohol use does not potentiate smoking progression over the short-term. Social factors may be important early in the smoking career and contribute to continued smoking and smoking progression. Social facilitation expectancies and alcohol use may be effective targets for prevention and early smoking intervention. Findings also highlight the heterogeneity of less than daily smoking in young adulthood and the shortcomings of broad classifications of "nondaily" smoking.