PROJECT SUMMARY / ABSTRACT In response to RFA-DA-22-046, HEAL Initiative: Harm Reduction Policies, Practices, and Modes of Delivery for Persons with Substance Use Disorders, we will conduct a 5-year, community-engaged research project that aims to extend and enhance the delivery of harm reduction services to hard-to-reach people who use opioids and/or stimulants, to reduce their risk of fatal and nonfatal overdose. The main objective of this research is to design, and pilot test a mobile health (mHealth) intervention comprised of a bundle of internet and mobile phone-based tools for reducing overdose risk. We will build upon a productive research partnership with an established, comprehensive syringe services program operating in 10 cities in Wisconsin to pursue 3 specific aims. First, using a prospective cohort study design and several innovative methods for capturing harm- reduction service engagement, psychosocial variables, and behavioral outcomes, we will characterize the most important mechanisms through which harm reduction influences overdose risk behaviors. Second, we will study the role of harm reduction service engagement in linking clients to sources of health care and addiction treatment services, making novel use of linkages between administrative datasets in partnership with state government agencies. Third, we will convene a Community Leadership Team consisting of people who use opioids and/or stimulants, and collaboratively synthesize our research findings to inform the development and implementation of an innovative strategy for engaging clients who face barriers to accessing harm reduction services. The community engagement activities will culminate in a single-arm pilot intervention trial that evaluates the feasibility and acceptability of a suite of internet and smartphone-based tools designed to improve access to safer drug use behaviors and equipment, fill knowledge gaps, and deliver social support through connection to trusted peer networks. This research is guided by the central harm reduction principle of ensuring that people who use drugs have a real voice in the creation of programs designed to serve them. It will generate a robust understanding of the important mechanisms through which harm reduction services reduce overdose risk, and test promising strategies for extending their reach to under-served clients.