Health inequities and disparities among various racial/ethnic minority, sexual minority, and rural populations are the focus of increasing national efforts. Three health problems disproportionately affecting these populations—HIV/AIDS, substance abuse, and trauma—deserve particular attention because of their harmful effects on health across the life span. To address these problems, our training program, the UCLA HIV/AIDS, Substance Abuse, and Trauma Training Program (HA-STTP), mentors and trains early career behavioral health scientists to conduct research using scientifically sound, culturally collaborative, and population-centered approaches. HA-STTP has been highly successful in training a diverse, productive, nationwide group of scholars. The program provides 2 years of training and mentorship to 20 (5 per year over 4 years) scholars. It is unique in its attention to traumatic stress as a form of dysregulation, particularly as experienced by underserved populations. Furthermore, our training program embraces a uniquely comprehensive, culturally grounded understanding of traumatic stress and its implications for substance abuse and HIV. HA-STTP advances scholars’ knowledge of the interconnections among substance abuse, HIV/AIDS, traumatic stress, and health disparities, particularly in underrepresented populations; provides intensive mentorship to support scholars’ research interests and career trajectories; capitalizes on a multidisciplinary, multiracial/ethnic network of expert faculty; and evaluates the program’s impact on scholars’ knowledge and productivity. By fostering the growth of scholars committed to conducting research with underrepresented populations that are disproportionately affected by HIV/AIDS, substance abuse, and traumatic stress, this program enhances nationwide efforts to diminish the prevalence of these problems and improve health and quality of life.