Varicella-zoster (VZ) virus causes two diseases, varicella and zoster. Because it was recognized that, after varicella, latent VZ virus persisted in the host and because varicella was thought to be a mild disease, there was little impetus in North America to develop a live VZ vaccine. More recently, it has become apparent that currently available methods of prevention and treatment of severe varicella are not always efficacious. With the development of a live attenuated VZ vaccine by Takahashi and the successful use of the vaccine in immunocompromised children in Japan, there has been an increased interest in North America in immunization against varicella. In this article the natural history of VZ infections and the immunologic reactions to VZ virus are described, and the development and use of VZ vaccine in Japan are reviewed. Plans for trials of VZ vaccine in immunocompromised North American children are outlined. The major questions concerning the large-scale use of VZ vaccine are presented.