PROJECT SUMMARY: Young people living with HIV (LWH) in the developmental periods from adolescence to emerging adulthood (ages 16-28 years), which we refer to as “youth and emerging adults” (YEA), have the lowest rates of engagement along the HIV care continuum (HCC) of any age group. This poor engagement is most pronounced for African American/Black and Latinx YEA-LWH, including sexual/gender minorities, those who use alcohol/drugs, and individuals from low socioeconomic status backgrounds. Yet, our knowledge of factors that promote or impede engagement along the HCC, and how they operate and interplay over time, is insufficient, for both limited occurrence behaviors (e.g., HIV care appointments) and repeated occurrence behaviors (e.g., ART adherence). These gaps limit innovations to policy and health services. The proposed prospective longitudinal study uses mixed methods and focuses on African American/Black and Latinx YEA- LWH both with and without viral suppression. The study uses quantitative, qualitative, biomarker (HIV viral load, drug use), and ecological momentary assessment approaches to uncover, describe, and better understand factors contributing to trajectories of engagement along the HCC over time, with precision, including from YEA-LWH’s perspectives. The study is grounded in Social Action Theory, a comprehensive social-cognitive/ecological model that allows for examination of changes in contextual influences (e.g., action contexts such as racism and homophobia; self-regulatory resources such as substance use), self-change processes (e.g., social interaction processes including social networks; motivational appraisals), and action states (e.g., attendance at HIV care appointments, substance use treatment [as needed], ART re-initiation). The study’s primary outcome is HIV viral suppression (VS). Two interconnected aims are proposed: to describe trajectories of engagement along the HCC (Aim 1), and guided by Social Action Theory, to identify, describe, and understand the contextual influences, self-change processes, and action states that impede or support the behaviors that comprise engagement along the HCC over time, including YEA-LWH’s perspectives on these factors (Aim 2). A Youth Advisory Board will be joined by scientific advisors to develop actionable recommendations based on study findings, which will be disseminated. Participants (N=270, half with HIV VS and half non-suppressed at enrollment) will be diverse African American/Black or Latinx behaviorally infected YEA-LWH recruited in New York City and Newark, NJ using a proven hybrid strategy. We will follow participants prospectively for 18 months using mixed methods, combining baseline and follow-up data (N ≈ 810 viral load observations), to assess the multifaceted domains expected to facilitate or impede engagement along the HCC, and YEA-LWH’s perspectives on their causes and meanings. The research team is expert in longitudinal mixed methods studies, HCC research, and AAB...