Cancer biology today encompasses a variety of specialties ranging from epidemiology to molecular biology and chemotherapy. One of the very active areas of research is that which supports the concept of a virus etiology for at least some forms of cancer. For several years it has been generally accepted that a variety of both DNA and RNA viruses can transform animal cells in vitro and that they are transplantable and induce tumors in vivo. The virus-transformed cells exhibit changes in growth characteristics typical of cancer cells, and commonly contain virus-specific nucleic acids and antigens. Characterization of tumor viruses and virus-cell associations are being explored by virologists, biochemists, epidemiologists, geneticists, molecular biologists, and immunologists. The nature of the complex problems involved requires the participation of experts in a variety of specialties. In this communication, we present an overview of the status of DNA tumor viruses as they relate to human cancer and to transformation in general. No human C-type RNA virus has yet been identified, although there are reports of primate C-type viruses. The potentially important human papovaviruses, human adenoviruses and the Herpesvirus group, including herpes simplex virus, cytomegalovirus, and the EpsteinBarr virus are discussed. The importance of the Herpesvirus group today as it relates to human neoplasia prompted us to emphasize this area.