Modified Project Summary/Abstract Section Young men are disproportionately impacted by HIV in the United States. Despite the perceived safety of romantic relationships, main partnerships account for a large proportion of new HIV infections in men, and this proportion is much higher amongst younger men. Men substantially reduce condom and PrEP use upon entry into a relationship, despite continued risk for many couples. However, healthy romantic relationships provide myriad benefits to individual health and wellbeing. Thus, we created 2GETHER, which integrates evidence-based relationship education (e.g., communication, conflict resolution) and HIV prevention (e.g., sexual health communication, behavioral/biomedical prevention), to optimize relationship functioning and reduce HIV risk among young men in relationships. 2GETHER demonstrated evidence of feasibility and acceptability, as well as preliminary effects on behaviors and relationship functioning, in a pilot trial. We have also adapted 2GETHER for online administration, consisting of 3 online didactic skills modules and 2 videoconference-based skills coaching sessions, and are evaluating its efficacy in a randomized trial. Despite its promise, we have identified several critical gaps in couple-based HIV prevention: 1) live skills coaching is burdensome and not cost-effective, 2) many partnered young men are unable to enroll partners and are thus excluded, and 3) newer relationships rarely enroll in trials, so we need to deliver relationship skills to single young men before relationship initiation. Our goal is to upgrade 2GETHER into a scalable and tailored intervention package for both single and partnered young men, consisting of: 1) a standalone relationship education and HIV prevention eHealth program (“eRE”) for single and partnered young men, and 2) two additional dyadic skills coaching sessions (for young men participating as dyads only). Aim 1: Evaluate the impact of the tailored 2GETHER intervention package on HIV incidence in high-risk young men. We will enroll 5,000 HIV-negative young men to test effectiveness of the entirety of the 2GETHER tailored intervention package on HIV incidence at 24-months post-intervention relative to control. We will also explore effects of particular tailored elements: (a) examine differential effectiveness between single and partnered young men; (b) among partnered men, analyze the additive benefit of dyadic skills coaching sessions beyond the automated eRE program. Aim 2: Explore differential effects across treatment arms among certain groups. Aim 3: Research the implementation of 2GETHER nationally to inform scale out and determine cost. Employing a Hybrid Type 1 Effectiveness–Implementation Trial design, we will use mixed methods to identify facilitators and barriers to implementation. We will measure key implementation determinants and outcomes through in-depth interviews with national stakeholders to inform future implementation strategies, and c...