This activity involves a realist literature review with community engagement to refine indicators of community-level barriers to accessing SUD treatment and overdose prevention services and associated downstream infection (e.g., HIV/HCV) risk, with a focus on emerging intervention strategies like telehealth, mobile units, and remote distribution sites (e.g. lockboxes, mail). This effort is designed to identify distinct community-level indicators and develop corresponding typologies characterizing contemporary SUD service access barriers and facilitators for all U.S. census tracts. Findings will inform both original research and refinements to the OEPS as described in JCOIN 203.