Moon and Sun Brothers: A group-based Prevention Intervention - The overall goal of this research is to reduce HIV-and related health variations among US male populations from Hispanic backgrounds. The quasi-experimental study will determine the effectiveness of a group intervention designed to reduce risk behaviors among Hispanic male sub-populations with the highest HIV risk. This quasi-experimental, longitudinal design has both an intervention and a control arm. Assessments will be conducted at -2 and 0 months, and follow-ups at 3, 6, and 12 months. Participants (N=360; n=180 per arm) will be recruited using time-location sampling (e.g., a venue-based strategy). The intervention at the heart of this study offers several novel features: it focuses on Hispanic men, socio-cultural risk factors shaping HIV susceptibility, and uses an approach centered on local involvement, such as civic participation and volunteering, as a means to foster health behaviors and group social support. The study will help identify effective interventions to prevent HIV among Hispanic male subpopulations at highest risk, but who remain understudied, and for whom HIV preventive interventions are severely lacking. This study capitalizes on an existing intervention and is the first of its kind to rigorously test, using a control arm, the intervention’s effectiveness among Hispanic men who do not exclusively identify as heterosexual or engage primarily in heterosexual relationships. The study applies the best possible scientific approach to obtain estimates of intervention effectiveness. If successful, the intervention can inform future interventions in the target population including its dissemination via local organizations and the Centers for Disease Control and Prevention. Ultimately, the study will help advance efforts to improve health across sociodemographic groups and thus, across the whole U.S. population. The project is led by San Francisco State University in collaboration with other academic partners and local organizations, the latter situated in each of the study sites. The research team is multidisciplinary with the depth and breadth of expertise in prevention science, along with extensive experience working with the targeted study population all of which required to successfully implement the project.