PROJECT SUMMARY/ABSTRACT The intersection of harm reduction and criminal legal systems (CLS) presents significant challenges and opportunities for public health research. Recognizing the critical role of CLS as a structural determinant of health, the National Institutes of Health (NIH) seeks to advance understanding and implementation of harm reduction strategies within this context. Through an administrative supplement to the existing Harm Reduction Research Network Coordination Center (HRRN-CC), this project aims to develop a comprehensive roadmap for future research that addresses these challenges and leverages opportunities. The HRRN-CC, funded by NIH, coordinates research across 10 projects focused on community-based harm reduction strategies. This supplement will build on existing efforts by focusing specifically on CLS. Aim 1 will facilitate the analysis and dissemination of measures related to CLS involvement and its intersection with harm reduction. The HRRN CLS Workgroup will provide technical assistance for analyzing common CLS metrics and support researchers in publishing findings. Aim 2 will identify opportunities for harm reduction strategies across various points of CLS involvement using the Sequential Intercept Model and systematic reviews of evidence-based practices. Aim 3 will seek consensus between harm reduction and CLS practitioners on priority research areas through a Delphi process, engaging both the HRRN’s Community Expert Council and JCOIN’s Board of Practitioners. Finally, Aim 4 will develop a future research agenda by synthesizing findings from the CLS metrics analysis, systematic reviews, and Delphi process, culminating in a translational report. Led by investigators Drs. Emmanuel Oga and Jessica Cance, with foremost CLS experts, Dr. Brad Ray and Leo Beletsky co-facilitating the HRRN CLS Workgroup, the project leverages significant expertise in harm reduction, data harmonization, and statistical methodology. Dr. Jason Williams will lead analysis activities, Dr. Kiersten Johnson will oversee systematic reviews, Dr. Sheila Patel will manage the Delphi process, and Dr. Cance will coordinate dissemination efforts. This project will provide a robust framework for integrating harm reduction strategies within CLS, addressing policy barriers, and enhancing the effectiveness of interventions. By charting a path for future research at the intersection of harm reduction and CLS, it aims to reduce health disparities and improve outcomes for individuals affected by substance use and criminalization.