The investigation of Mg in animals, including humans, has lagged behind that in bacteria and yeast. This is largely because of the complexity of the systems involved, but also because of the impression within the field that Mg was maintained at high levels in all cells and was unchanged by external influences. Only in the last 25 years has a series of reports begun to challenge this view, with new methodologies finding that free Mg content is maintained at levels where changes might influence cellular metabolism.