The discovery and development of more than a dozen drugs in the past 15 years for the treatment of AIDS offer an excellent example of progress in the field of rational drug design. At this time, the principal targets are reverse transcriptase and protease, enzymes encoded by the human immunodeficiency virus. The introduction of protease inhibitors, in particular, has drastically decreased the mortality and morbidity associated with AIDS. This review presents the methods used to develop such drugs and discusses the remaining problems, such as the rapid emergence of drug resistance.