This volume discusses our present knowledge of the role of viruses in the causation of cancer, both in humans and in animal model systems. The fourteen chapters consider the molecular biology and the biology and epidemiology of those viruses which appear to have a role in human cancer, namely hepatitis B virus, Epstein-Barr virus, papillornaviruses, herpes viruses and the Human T-Cell leukaemia virus and discuss various aspects of other viruses which are oncogenic in animal systems, and have provided important information as to the mechanisms of malignant transformation.