ABSTRACT We have one specific aim: Train postdoctoral fellows who will be qualified and well-positioned to become leaders in the development and implementation of substance use research in tobacco control, public health, policy, and clinical practice. Specifically, we propose a postdoctoral fellowship program that will attract individuals with a strong commitment to transdisciplinary research from a variety of medical, biological, social, behavioral, and policy sciences to help build the next generation of scientific leaders in tobacco control and related substance use. Tobacco remains the leading preventable cause of death the U.S. and disproportionately burdens people with substance use disorders. The need for tobacco control expertise continues to grow with demand for research to inform FDA regulation of tobacco products, the implementation of the WHO Framework Convention on Tobacco Control, and state and local tobacco control efforts to address the rapidly evolving tobacco product environment. Considering the high rates of tobacco co-use with other substances, our program is expanding on points of synergy between tobacco, cannabis and opioid research. This includes training in: 1) Research on tobacco and cannabis co-use and cannabis health effects to inform policy and develop evidence-based interventions and 2) Research on industry behavior that has driven both the tobacco and opioid epidemics and is likely to become increasingly important with cannabis legalization. We provide this training through integrated transdisciplinary coursework in tobacco science, tobacco policy, addiction, and biostatistics, combined with mentored research with two faculty from different disciplines. Fellows complete a course in which they prepare, and submit grant proposals to funding agencies for their third year of fellowship funding. Research mentors include 31 faculty members from all four schools at UCSF (Dentistry, Medicine, Nursing, and Pharmacy) with active research programs in tobacco policy, marketing, addiction, health effects of smoking and secondhand smoke exposure, emerging tobacco, cannabis and vaping products, cessation, cannabis substitution for opioids, and analyses of tobacco and opioid industry documents. The diverse fellow backgrounds and interests in the Center for Tobacco Control Research and Education provides a cooperative environment that creates continuous opportunities to learn from and appreciate work in other disciplines. Each fellow creates an Individual Development Plan that is reviewed by both mentors and the Fellowship Committee. The Committee selects fellows from a competitive national pool: we average 29 applications per year for 3 slots. Between 2016-2021, our fellows produced 135 papers in 68 different journals. Since 2001, we have trained 92 fellows (62 funded by NIH training grants since 2006); 93% of our R25/T32 graduates have research careers in academic, government and nongovernmental institutions, and 54% are academic f...