Studies in vitro of oncogenic agents have attained an ever increasing importance mainly because of the precision and ease by which events at the cellular and sub-cellular level can be followed. This article will examine the relevance of such data for the understanding of a much more complicated process — the development of spontaneous malignant tumors in vivo. This seems particularly important in view of the enormous current interest in the possibility that human cancers may be caused by RNA viruses — a hypothesis largely resting on observations in vitro.