Microbicides are chemical agents used topically by women within the vagina in order to prevent infection by HIV and potentially by other enveloped viruses and sexually transmitted pathogens. Prototype microbicides are designed to be inserted prior to each act of sexual intercourse and could also be contraceptive, although most current potential microbicides are not. Several proof-of-principle phase III trials of candidate microbicides are currently in progress or are shortly to commence, and a definitive answer to their efficacy and safety is anticipated by 2008.