The current interest in the virus theory of cancer is illustrated by the number of detailed reviews of the role of viruses in the etiology of some animal cancers and their possible role in human cancer.1-6 This review will be limited to certain historical data and to significant new data on certain tumor viruses, as well as recent discussions of the problems of human tumor viruses available since the previous reviews. The purpose of this review will be (1) to describe the historical background of the virus theory of cancer; (2) to present the theories on how a virus may convert a normal cell into a malignant cell. For this purpose, the current concepts of a normal cell, a cancer cell, and a virus will be described. Further, the purpose will be (3) to describe certain significant new data concerning virus-induced neoplasia in cells cultivated in vitro and in