Substance use peaks during emerging adulthood, the age period from the late teens to the early 20s. And while cross-sectional studies have established that cigarette smoking and alcohol use are highly correlated, very little is known about the longitudinal relations between cigarette smoking and alcohol use during this stage of development. Emerging adulthood is characterized by individual variability and includes a number of life transitions potentially relevant to substance use, including the transitions into an out of college and an individual's 21st birthday, when alcohol use is no longer an illegal activity But analyses that use variable- centered approaches, such as traditional regression, rely on the assumption that the longitudinal co- occurrence of cigarette smoking and alcohol use are the same for all participants, thus ignoring this individual heterogeneity. The purpose of this research is to develop a better understanding of the individual co-occurring patterns of cigarette smoking and alcohol use throughout emerging adulthood. The analyses will be conducted using existing data from the UT Experience! study, a cohort study of 2,247 high school graduates recruited during the summer of 2004, prior to college matriculations, who were followed for 6 years (10 waves of data collection). At Wave 1, the sample was 60% female, 55% identified as Caucasian, 18% Asian, 16% Hispanic, 4% African American, and 7% identified as another racial/ethnic group. Data were collected online through a secured website. The proposed study will use two person-centered methodological approaches, growth mixture modeling and latent transition analysis, to 1) test the longitudinal co-occurrence of cigarette smoking and alcohol use throughout emerging adulthood, and 2) determine how life transitions impact the co- occurrence of cigarette smoking and alcohol use throughout emerging adulthood. PUBLIC HEALTH RELEVANCE: The use of cigarettes and alcohol in emerging adulthood (e.g., ages 18-25) is associated with significant consequences. Given the heterogeneity of substance use throughout emerging adulthood, it is important to understand the individual patterns of co-occurring use that have the highest potential for future consequences and whether there are stages within emerging adulthood where these co-occurrence patterns shift. It would then be possible to target interventions to those individuals at highest risk at the time periods that are most likely to influence their future substance using behaviors.