Ribavirin (1-beta-D-ribofuranosyl-1,2,4-triazole-3-carboxamide) is a broad-spectrum antiviral agent whose molecular mode of action remains remarkably controversial. This antiviral agent was approved by the U.S. Food and Drug Administration in 1986 for use as an aerosol for infants with serious infections due to respiratory syncytial virus. Ribavirin is and has been under clinical investigation for activity against a variety of viral illnesses, including those due to influenza virus, Lassa fever virus, Hantaan virus, and human immunodeficiency virus (HIV). There has been a great deal of clinical interest in the utilization of ribavirin for treatment of infections due to HIV. It has been reported to slow the development of AIDS in HIV-infected patients. We describe here the major mechanisms of action of this newly licensed antiviral agent.