The transformation of a normal into a tumor cell is not caused by a single molecular event, but is the consequence of several simultaneous or consecutive molecular processes, which lead to a variety of changes in the structure and the metabolism of the cell. Investigations with the Rous sarcoma virus show that a single gene is primarily responsible for these changes that is coding for a phosphoprotein which, however, is multifunctional. The biochemical and biologic events which initiate and maintain the transformed status of the cell involve mainly the cytoplasma membrane. At both the outer and the inner surface of the cell membrane dramatic changes occur which influence the cell structure, permeability of the cytoplasma membrane, and the intracellular metabolic pathways. Most probably, these transformation-associated events are also involved in cell proliferation under physiologic conditions. In the tumor cell, however, they are not further regulated physiologically, with the consequence of an uncontrolled and incessant cell division.