We stand today on the threshold of a new understanding of cancer. Primarily through the powerful tools of molecular biology, unified hypotheses explaining the origins of the disease are emerging and rapidly being validated. This volume, which presents the latest findings from laboratories throughout the world on the role of RNA tumor viruses in cancer, is a celebration of these achievements and a prediction of further progress leading ultimately to the control of the disease. It is important in this context to recall the natural history or life cycle of RNA cancer virology. From the earliest days of the science, when viruses were first recognized as distinct biologic agents of etiologic significance, their role in cancer was proposed and hotly debated. The critical early discoveries, even those made as recently as 25 years ago, were met with rejection; not skepticism or cautious restraint, but outright rejection. During the 60's, there was a gradual acceptance of the association between viruses and cancer, the result of landmark studies in experimental systems, and this led to a frenzy of activity in the field. There followed another period of doubt and uncertainty, due to the difficulty in attempting to apply directly, and in retrospect inappropriately, the tenets of infectious disease to human cancers, only to have the field resurrected, revitalized and redirected by the explosion of progress in molecular biology and genetics.