The first step in the generation of a malignancy seems to be a transformation in the genetic apparatus of a single cell. The ultimate nature of the cancer which appears is a result of the interaction of that change with the control and regulatory apparatus of the whole animal. It appears that the primary cellular change which may be induced by physical, chemical or biological agents (or a combination of them) may be something which is common to all carcinogenesis. The nature of that primary change and how it may result from the action of viruses, chemicals, and radiation (or interaction between them) is the subject of this discussion.