Oncogenes are genes with a proven cancer association that appear to function primarily in the regulation of cellular proliferation and differentiation. The preliminary work leading to their discovery, their association with retroviruses, the retrovirus life cycle, and the use of retroviruses to find and characterize oncogenes is discussed in detail. Chromosomal abnormalities are discussed using Burkitt's lymphoma and themyconcogene as prototypes. Finally, the normal (nonmalignant) functioning of oncogenes is discussed in terms of embryogenesis and hepatic regeneration.