Although the number of new infections has declined recently, women still constitute almost half of the world's 34 million people with HIV infection, and HIV remains the leading cause of death among women of reproductive age. Prevention research has made considerable progress during the past few years in addressing the biological, behavioral, and social factors that influence women's vulnerability to HIV infection. Nevertheless, substantial work still must be performed to implement scientific advancements and to resolve many questions that remain. This article highlights some of the recent advances and persistent gaps in HIV prevention research for women and outlines key research and policy priorities.