Recent years have witnessed the increasing adoption of an evidence-based public health approach in HIV prevention science, yet this approach is limited by lack of consensus about what counts as "evidence." In 2007, the San Francisco AIDS Foundation (SFAF) launched an initiative to engage HIV/AIDS organizations, researchers, program implementers, policy makers, and funders in a robust discussion of what really constitutes "evidence" in evidence-based HIV prevention. The core of this initiative is a series of panel discussions about the gathering and interpreting of evidence for establishing efficacy and effectiveness in HIV prevention--evidence that is then used to inform decisions about which interventions to implement, fund, and scale up.