Abstract Obesity and tobacco use among adolescents and young adults remain two of the most pressing public health concerns in the U.S.; both are linked to pervasive declines across health domains. The transition from adolescence to young adulthood – emerging adulthood - presents a critical period for examining these health- risks; problematic substance use often emerges during this developmental stage and obesity status during this period is predictive of chronic obesity. Research on co-occurring obesity and substance use points to a strong link between obesity and tobacco use; however, the pathways and processes underlying this co-occurrence are unclear. Furthermore, it is unknown whether links between obesity and tobacco use extend to e- cigarette/vaping use, which has surged in popularity in the past decade. The current study seeks to identify developmental pathways and risk processes to co-occurring obesity and tobacco use (cigarette smoking, e- cigarette/vaping use) during emerging adulthood. The study aims to collect longitudinal health-risk survey data (five timepoints over two years) from a probability sample of N = 1,450 young adults (18-24 years) enrolled as undergraduates at California State University, Long Beach (CSULB). Participants will be recruited from randomly sampled classes and asked to complete five online surveys over a two-year period. The survey includes questions on weight status, nutrition, physical activity, self-regulation, social relationships, psychosocial adjustment, and substance use. Given that the transition from adolescence to young adulthood is characterized by significant life changes, along with neurocognitive and socio-emotional challenges, key aspects of Steinberg’s Dual-Systems Model, Rimal and Real’s Theory of Normative Social Behavior, and Lazarus and Folkman’s Transactional Model of Stress and Coping frame the study’s conceptual model. Key study aims include: (1) determine the extent to which obesity and tobacco use co-occur during emerging adulthood and identify the pathways (obesity → tobacco use; tobacco use → obesity) to co-occurrence; (2) identify processes underlying co-occurring obesity and tobacco use, including biobehavioral (impulsivity, sensation-seeking), socio-contextual (social acceptance, weight stigma, deviant peer affiliation), and psychosocial (depression, social anxiety, perceived stress) components; and (3) evaluate whether obese emerging adults from specific ethnic/gender groups are at higher or lower risk of engaging in cigarette smoking and/or e-cigarette/vaping use. As both obesity and tobacco use place severe burdens on the health care system, identifying pathways and processes by which obesity is related to tobacco use in emerging adulthood is likely to have significant implications for prevention and intervention efforts aimed at mitigating co-occurring health-risks earlier in the lifespan.