Microbicides are biomedical products being developed to protect people against sexually transmitted infections, including HIV. Some microbicides are being designed as vaginal products for women, and others would be rectal products for either men or women. Several candidate microbicides have been developed and tested over the past two decades. Yet the results of clinical trials have been largely disappointing. Just one trial provided evidence that a vaginal microbicide could protect against sexual transmission of HIV. The results of a confirmatory trial are underway. Should it provide confirmation, several years would still be needed before a product is available in sufficient quantities.