The chemotherapy of virus infections has definitely come of age. There are now 15 antiviral agents that have been formally licensed for the treatment of human immunodeficiency virus infections (zidovudine, didanosine, zalcitabine, stavudine, lamivudine, abacavir, nevirapine, delavirdine, efavirenz, saquinavir, ritonavir, indinavir, nelfinavir, amprenavir, lopinavir) and several others, such as tenofovir disoproxil, emtricitabine, capravirine, emivirine, T-20 (pentafuside) and AMD3100 (bicyclam) are under clinical development. Lamivudine has been approved, and several other compounds (such as adefovir dipivoxil, emtricitabine and entecavir) are under clinical development, for the treatment of hepatitis B virus infections. Among the anti-herpesvirus agents, aciclovir, valaciclovir, penciclovir, famciclovir, idoxuridine, trifluridine and brivudin are used in the treatment of herpes simplex virus and varicella-zoster virus infections, and ganciclovir, foscarnet, cidofovir, fomivirsen and maribavir (the latter in the developmental stage) are used in the treatment of cytomegalovirus infections. Following amantadine and rimantadine, the neuraminidase inhibitors, zanamivir and oseltamivir, have now become available for the therapy and prophylaxis of influenza virus infections, and so is ribavirin for the treatment of respiratory syncytial virus infections and the combination of ribavirin with interferon-alpha for the treatment of hepatitis C virus infections.