The development of new techniques for measuring intracellular free Mg2+ during the 1980s has provided investigators with the tools needed to produce new insights into the regulation of cellular magnesium. Within the limits of this technology, it appears that all mammalian cells maintain free cytosolic Mg2+ levels within the fairly narrow range of 0.25-1 mM. While transport mechanisms and sequestration within cellular organelles will contribute to this regulation, it is binding of Mg2+ to an as yet poorly defined system of buffers that is largely responsible for determining the set point of this regulation. The lack of an adequately Mg2+-selective ionophore remains an impediment to progress in this area.