Publisher Summary The oncogenic potential of many animal viruses is now well established, and the use of tumor viruses now dominates efforts to dissect the mechanisms of tumorigenesis. Although several themes appear to unite oncogenesis by diverse viral agents, and each family of tumor viruses has important distinctive features, it is the retroviruses that have provided the most coherent and penetrating view of tumorigenesis. Three features of retroviruses account for this sentiment. First, the oncogenes of retroviruses have proved exceptionally accessible to definition and study, and as a consequence, they have provided first glimpse of enzymatic mechanisms responsible for neoplastic transformation. Second, the diversity of retrovirus oncogenes has provided a rich set of oncogenic agents whose versatility far exceeds that of DNA tumor virus oncogenes, and whose tumorigenic capacities provide separate experimental models for most major forms of malignancy. Finally, oncogenes appear not to be indigenous components of retrovirus genomes, but instead have been transduced from normal genetic loci of the vertebrate hosts in which retroviruses replicate.