This paper summarizes the extensive body of research on the state of girls' education in the developing world today; the impact of educating girls on families, economies, and nations; and the most promising approaches to increasing girls' enrollment and educational quality. The overall conclusions are straightforward: educating girls pays off substantially. While challenges still exist, existing research provides us guidance on how to make significant progress. More than half of the countries that are not on track to reach the goal of universal primary education are also those worst affected by HIV/AIDS. The impact of the disease on girls' schooling is more acute in countries with the highest HIV prevalence rates, where girls' enrollments are declining due to care-giving responsibilities or their own infection. Girls' enrollments are more stable in countries with lower prevalence rates. Today, an increasing body of research shows that more-educated people, especially youth, are less likely to engage in risky behavior and contract HIV.