The containment of cell growth is at the core of the homeostatic regulation of metazoans, and considerable progress has been made in the understanding of how this is achieved. Most knowledge comes from the isolation of molecu­ les with positive and negative regulatory effects on cell proliferation, and most emphasis so far has been on these molecules. Some of these molecules are already available for therapeutic purposes, and others look promising in this respect. This volume gives examples of such approaches. The understanding of the control of cell growth is also fundamental to grasp phylogenic and ontogenic development. Why organisms have developed increasingly sophisticated mechanisms that control their size and that of their organs, how different cells originate, some destined for renewal and repair, others for specialized functions in a postmitotic state or evolving through division, others like the germinal cells waiting for the signal to start another organism. There is one mechanism of growth containment, however, about which we know very little. It concerns the structural characteristics of the cell, i.e. the relationship between structure and function. How structure can change the response to identical signals. The positive and negative growth regulators may be conserved, but the structure and organization of the genetic material and of other cell components differ widely and are responsible to a great extent for the differences in cell proliferative behaviour.