ABSTRACT Substance use disorders (SUD) are common, chronic, and associated with substantial death and despair but can be treated. American Indian/Alaska Native (AI/AN) populations are vulnerable to health and healthcare disparities and increased likelihood of SUD due to colonialism, historical trauma, and structural racism. Understanding substance use and its treatment among AI/AN Veterans would fill a major knowledge gap and provide needed foundation for development and testing of a culturally appropriate SUD intervention that can be built on strength and resilient factors among AI/AN Veterans and delivered in clinical settings. Guided by Public Health Critical Race (PHCR) praxis, the mixed-methods parent R01 provides a unique and synergistic opportunity to conduct needed foundational clinical epidemiology and in- depth qualitative work to inform development of an intervention that can be tested in a Type I Hybrid design. The proposed supplement capitalizes on this rich foundation and substantial upfront investment from NIDA to add an unprecedented and community-engaged line of inquiry into substance use and its treatment among AI/AN Veterans and support culturally appropriate intervention development. The overall goal of the proposed supplement is to build on the resources developed through the parent study to conduct research in partnership with AI/AN community members and guided by PHCR to provide the foundation for developing a Hybrid Type I study of a culturally appropriate, strengths-based intervention. The specific aims, all of which will be guided by community partners, include: Aim 1) Describe the prevalence of unique substance use disorders among AI/AN Veterans, overall and relative to White Veterans, and identify multilevel factors associated with SUD among AI/AN Veterans; Aim 2) Describe receipt of and multilevel factors influencing evidence-based treatment (e.g., specialty addictions treatment and/or receipt of effective pharmacotherapies) among AI/AN Veterans with SUD, and compare likelihood of receipt to those of White Veterans with SUD; Aim 3) Understand experiences of SUD, their treatment, and multi-level lived experiences that influence these among AI/AN Veterans through in-depth qualitative interviews with ~40 AI/AN Veterans. The proposed community-partnered innovative foundational study will be the first to our knowledge to describe the prevalence of SUD and its treatment, as well as multi-level factors that result from structural racism to influence these experiences, among AI/AN Veterans. It is also the only study to our knowledge to obtain in-depth qualitative information to understand the lived experiences of AI/AN Veterans with SUD and how they influence SUD treatment and the multi-level factors that influence both. Together the planned activities will provide foundational data and community engagement we need to design a community-engaged, culturally appropriate intervention to decrease SUD and increase SUD treatment am...