Information about the involvement of Mg2+ in all biochemical processes that participate in cell proliferation is reviewed in order to define the role of this divalent cation in normal and pathological growth. The lack of conclusive data about cell Mg2+ homeostasis does not suggest any definitive model for its role in the control of cell proliferation. On the other hand, new important information about its absolute requirement in crucial steps of cell activation that can, beside other functions, trigger cell division, strongly support the involvement of Mg2+ in the control of cell proliferation. Studies on the growth of cells in vitro, however, while confirming the indispensible requirement for Mg2+ in extracellular media, do not completely clarify the mechanism(s) or the exact phase/point of the cell cycle where Mg2+ exerts its regulation. Furthermore, the observation that tumour cells grown in culture are influenced by external divalent cations confirms the involvement of Mg2+ in cancer as well as in normal cell proliferation. The proposed explanations (theories, hypotheses) are described and discussed.