This report by the Global HIV Prevention Working Group offers a new analysis that examines the future course of the epidemic with and without a greatly scaled-up prevention response; surveys the latest evidence on HIV prevention access; reviews the experience in countries where such barriers have been overcome; and closes with a series of urgent recommendations to bring the promise of HIV prevention to the countries and communities that need it the most. As the report shows, even in the midst of global failure to make optimal use of available prevention strategies, a number of countries, including some of the world’s poorest, have made tangible progress in reducing the number of new HIV infections through implementation of comprehensive HIV prevention efforts. Strong evidence and replicable models exist for HIV prevention scale-up, underscoring the need to move beyond localized pilot projects to broad-based, comprehensive national programs. If comprehensive HIV prevention were brought to scale, half of the infections projected to occur by 2015 could be averted.