The AIDS pandemic will enter its fiftieth year in 2031. Despite much progress, there are thirty-three million infected people worldwide, and 2.3 million adults were newly infected in 2007. Without a change in approach, a major pandemic will still be with us in 2031. Modeling carried out for the AIDS 2031 project suggests that funding required for developing countries to address the pandemic could reach $35 billion annually by 2031-three times the current level. Even then, more than a million people will still be newly infected each year. However, wise policy choices focusing on high-impact prevention and efficient treatment could cut costs by half. Investments in new prevention tools and major behavior-change efforts are needed to spur more rapid advances. Existing donors, middle-income countries with contained epidemics, philanthropists, and innovative financing could help bridge the likely funding gap.