The regulation of cell proliferation in multicellular organisms is a complex process, which is primarily regu-lated by external growth factors provided by surrounding cells. Once induced to proliferate, the passage through the mitotic cell cycle is directed by the components of the so called cell cycle machinery. Over the last years research on growth factor signal-transduction and the components of the cell cycle system has lead to a detailed knowledge of the mechanisms by which growth factors transmit their signals and of the relationship between the components of the cell cycle machinery. The remaining question how the growth factor mediated signal-transduction cascades couple with the cell cycle regulators has recently been a focus of interest.