Our research team is supported by the Avenir Award from the National Institute on Drug Abuse (#1DP2DA046856-01). The goal of this project is to develop causal inference methodology combined in novel ways with network science to solve critical challenges at the nexus of HIV and substance use for network-based studies of HIV treatment and prevention among People Who Use Drugs (PWUD). This project aims to advance HIV prevention and treatment research by contributing new methodology to study causal mechanisms in networks of people who use drugs at high risk for HIV infection. Limited methodological research has been done to quantify causal individual, disseminated and composite effects in non-randomized or observational settings using agent based models (ABMs) and network science approaches. In studies with more general forms of dissemination (e.g., between networks), the causal effect of interest can be difficult to identify. Claims of causality are further complicated by such issues as homophily, i.e., similar individuals tend to be connected, and confounding both at the individual- and network-levels. Without these developments, we cannot quantify the full impact of interventions on the determinants of HIV risk and the HIV care continuum among PWUD networks.