STD prevention has always included seeking and treatment of sexual partners of infected persons as a core element of reducing transmission of disease. However, traditional prevention strategies primarily focus on disease as a consequence of partnership formation. The biological, psychological, and social processes of partnership formation are less well understood. This session is based in the premise that formation of sexual partnerships is not a random process, and that asking questions about why people choose specific sexual partners could enhance prevention strategies. The first part of this session will explore biological reasons why people choose their sexual partners with focus on the role of the brain in partner selection. Next, the session will explore the relationship between how people cluster in groups and the sexual partners they choose. Finally, a discussion about how these sexual partnering issues at an individual and group level pertain to STD prevention efforts.