PROJECT SUMMARY/ ABSTRACT The purpose of this timely and significant K01 is to provide the PI (Dr. Bunting) with the training and expertise necessary to become an independent investigator of substance use research, specializing in the development of interventions that improve health outcomes of vulnerable populations with a specific focus on polysubstance use involving opioids. Despite the prevalence of polysubstance use and its association with high-risk outcomes such as overdose, significant gaps in our understanding persist in part due to limitations of existing substance use screening and assessment tools. The ability to accurately measure polysubstance use among high-risk populations is a critical step towards developing effective strategies for improving substance-related health outcomes of this population. The overall goal of the proposal is the development of a novel polysubstance assessment tool and identification of correlates and motivations of polysubstance use behaviors among high- risk vulnerable populations. The long-term goal of this line of investigation is to develop interventions targeted to polysubstance use behaviors in vulnerable populations as an independent investigator of substance use research. To achieve this goal, the proposed award has been designed to augment the PI’s existing training in sociology and health services through a comprehensive plan that will enable her to: (1) obtain methodological skills specific to instrument development and validation, (2) enhance her ability to conduct independent research including the recruitment and retention of vulnerable populations and remote study management, and (3) develop focused skills in health services intervention development specific to vulnerable populations (i.e., persons who use substances, justice-involved). These skills will be developed at the high-impact training environment of the New York University Grossman School of Medicine with a collaborative mentoring team of nationally renowned experts. The training plan is strategically complemented by a rigorous mixed-method study. Building on prior training while expanding to new skills, the independent line of research aims to: (1) develop and test the psychometric properties of a novel assessment tool to measure polysubstance use for high-risk vulnerable populations, (2) examine the predictive validity of the novel polysubstance use assessment tool by examining risk behaviors over a 6-month period in a high-risk vulnerable population, and (3) acquire preliminary data regarding the development and acceptability of intervention strategies for polysubstance use populations. By the conclusion of this award, the PI will be positioned as a rising leader in polysubstance use research and positioned to excel in a career of polysubstance use interventions through future R-mechanism funding opportunities.