Abstract：Viruses and cancers have been important subjects for research in biomedical sciences for many years. It is nowestimated that at least 20% of human cancers worldwide have an underlying viral etiology. In this article oncogenesis by retroviruses will be reviewed in details. In terms of disease, retroviruses can be divided into two classes based on howrapidly they cause disease. Acute transforming retroviruses induce tumors rapidly, while non-acute retroviruses induce disease more slowly. Their mechanisms of carcinogenesis are different, and this reviewwill address them separately. Acute transforming retroviruses contain oncogenes which endow viruses with the ability to induce tumors rapidly. However, the common mechanism by which non-acute retroviruses induce cancers is insertional activation of cellular proto-oncogenes. The study of oncogenesis of retroviruses identified several important principles of carcinogenesis including activation of proto-oncogenes and inactivation of tumor suppressor genes that turned out to be important for non-viral cancers of humans as well.