HIV prevention remains a top priority for the global health community, especially in sub-Saharan Africa, where 70 percent of the world’s estimated 25 million HIV-infected people currently live. Information campaigns to discourage risky sexual behavior have had limited success. Researchers are exploring the impact of using financial incentives to encourage safe sex in order to stem the spread of HIV. The evaluation in Tanzania will focus on high risk populations, such as female sex workers, to see if cash payments conditional on testing negative for HIV and other sexually-transmitted infections can help reduce risky behavior.