Abstract of Diversity Supplement The proposed Diversity Supplement is designed to promote diversity in the scientific community through the training of Samantha Schiavon, M.A., a Latinx clinical psychologist with experience conducting substance use disorder treatment research among vulnerable and racial/ethnic minority populations. Ms. Schiavon has obtained a strong research background through her pre-doctoral training in the Medical/Clinical Psychology Doctoral Program at the University of Alabama at Birmingham and will continue to advance her research competencies as a research postdoctoral fellow at the University of California San Diego (UCSD). Through this supplement, Ms. Schiavon will build her research capacities through examining the impact of stigma on motivations to initiate medication for opioid use disorder (MOUD) treatment among persons who inject drugs (PWID) within Mexico and the U.S. She will train under the mentorship of Dr. Steffanie Strathdee (primary mentor and PI on parent grant), a world-renowned infectious disease epidemiologist focused on HIV prevention research in developing countries, Dr. Thomas Patterson (co-mentor), known for his HIV/AIDS prevention interventions along the Mexico-U.S. border region, and Dr. Laramie Smith (co-mentor), an expert in stigma research among PWID. Through secondary data analysis, this supplement will expand upon the aims of the parent grant to identify cross-cultural differences between Mexico and the U.S. in MOUD and substance use disorder stigma as potential barriers to MOUD treatment initiation intentions among PWID. The specific aims are to: 1) to examine country of primary residence (Mexico v. US) as a moderator between anticipated substance use disorder stigma and intentions to initiate MOUD treatment; 2) to assess differences in anticipated MOUD treatment (i.e., buprenorphine/suboxone vs. methadone) stigma by country of primary residence (Mexico v. US); and 3) to examine anticipated MOUD stigma as a mediator between country of primary residence (Mexico v. US) and MOUD treatment initiation intentions. These results will inform the development of culturally responsive interventions to reduce stigma as a barrier to MOUD treatment initiation and will serve as the foundation for Ms. Schiavon’s future K23 proposal. Therefore, this supplement will not only provide crucial support to enhance Ms. Schiavon’s training throughout her postdoctoral fellowship at UCSD, but will also facilitate her transition to becoming an independent clinical researcher through a K23 award focused on reducing barriers to MOUD treatment initiation among international populations of PWID.