This report from the Global HIV Prevention Working Group (PWG) focuses specifically on behavioral HIV prevention. The report surveys what we know about the effectiveness of behavior change strategies, what we still need to learn, and what we need to do to advance such efforts in coming years. Based on a comprehensive review of hundreds of studies of behavior change for HIV prevention, we find that the evidence base for behavioral HIV prevention is robust, with multiple studies documenting the effectiveness of interventions in numerous settings, among diverse populations, and throughout the course of the epidemic. Our review also indicates that the evidence base is not yet complete, and that important gaps and limitations remain in our knowledge about what works. Maximizing the effectiveness of prevention efforts requires that these limitations be acknowledged and addressed.