If tumor viruses did not exist in nature they might have been created by scientists interested in basic mechanisms of develop- ment, differentiation, and tumorigenesis. In contemporary euka- ryotic cell biology tumor viruses playa similar role to that which bacteriophages once had for the molecular biology of prokary- otes. Tumor viruses provide extremely useful probes for the above cellular processes since their life cycle is genetically pro- grammed and can be followed at DNA, RNA, and protein levels. The experimental systems reviewed in this volume utilize a wide variety of viruses. A comprehensive introduction to this field has recently been published in the volumes of Molecular Biology o/Tumor Viruses: DNA Tumor Viruses, 2nd edition, edited by J. Tooze; and Molecular Biology o/Thmor Viruses: RNA Tumor Viruses, 2nd edition, edited by R. Weiss, N. Teich, H. Varmus, and J. Coffm, by Cold Spring Harbor Laboratories in 1980 and 1982. Polyoma and SV40 viruses (see the chapter by A. Levine) and adenoviruses (see the chapter by W. Doerfler) are double- stranded DNA-containing viruses. Polyoma and SV40 are struc- turally related viruses which contain a genome of approximately 5 kilo basepairs, while the DNA of adenovirus is about 7 times more complex. These DNA tumor viruses are understood at a genetic and molecular level which is comparable to our know- ledge of A and T4 bacteriophages. Retroviruses, the subject of the remaining four chapters, con- tain a single-stranded RNA genome of 5-8 kilobases.