Oncogenes play a major role in the control of proliferation in animal cells. Because the growth of cell populations is regulated by the stimulatory and inhibitory growth factors in the environment, we have attempted in this review to relate the function of oncogenes to the mechanism of action of growth factors. This correlation allows a description of the cell cycle that rests on a molecular basis and in which protooncogenes figure prominently as the cellular messengers of the environmental growth stimuli.