New tools to prevent HIV are urgently needed to stem the estimated 2.6 million new HIV infections that occur worldwide each year. Recent research has shown that a new approach called pre-exposure prophylaxis, or PrEP, can reduce the risk of contracting HIV in gay and bisexual men at high risk of infection, when combined with existing prevention measures. Ongoing studies are evaluating this approach in other populations at risk for HIV, including heterosexuals and injection drug users.