Use of methamphetamine (meth) is a growing problem in the US. This growing public health crisis requires expanded research to explore its reach, correlates, and impact, including on HIV. Estimates of the incidence and persistence of meth use is needed, as well as research on how it co-occurs with other drug use (i.e. polysubstance use), its network- and neighborhood-correlates, and its impact on HIV transmission. Research Plan. In Aim 1, we will characterize meth use in an established cohort, including co-occurring use with other drugs (i.e. polysubstance use) and use over time (e.g., incidence, persistence). In Aim 2, we will identify network-level (e.g. network size, network turnover) and neighborhood-level (e.g., social cohesion, disorder) correlates of meth use. In Aim 3, we will assess how meth use impacts HIV transmission through HIV prevention (e.g. PrEP adherence, condom use), HIV treatment (e.g. ART adherence, viral suppression) and biological vulnerability (e.g. rectal cytokines). The ongoing Neighborhoods and Networks (N2) Cohort Study (R01MH112406; PIs: Duncan & Schneider) provides an ideal opportunity to conduct the proposed study. N2 includes 617 participants living in Chicago. Data being collected include meth use and other drug use at multiple cycles, multiple social network typologies, in-depth assessments of neighborhoods, and HIV-related prevention and treatment behaviors. The proposed study will use N2 data to conduct Aims 1-3. The results of this study will inform the development of an R34 proposal to develop and test an intervention that addresses meth use and HIV. Team. Investigators with expertise in meth use, HIV, social network analysis, spatial epidemiology, immunology, integration of biological and behavioral research, and mixed methods research will conduct this research together.