During the last 10 years, there have been major advances in the understanding of varicella-zoster virus and the diseases it causes. The molecular biology of the virus is being unraveled with the aid of new molecular technologies. Varicella, usually a benign manifestation of primary infection, and zoster, a result of reactivation of latent virus, can cause considerable morbidity in patients with immune impairment. Antiviral drugs, especially acyclovir, ameliorate severe infections but still have little role in the treatment of most normal patients with varicella or zoster. Varicella can be prevented when necessary by patient isolation and passive prophylaxis with varicella-zoster immune globulin. An experimental live vaccine also prevents varicella, but problems regarding its virulence for immunosuppressed patients and the durability of the protective response are still being addressed.