Recent findings regarding roles for magnesium in immunocompetence confirm and extend previous knowledge of its participation in natural and adaptive immunity. The detrimental effects of severe magnesium deficiency have been confirmed. There is better comprehension of how magnesium relates to mechanisms that control cellular activities and regulate interactions among cells that affect immune functions. Insight has been gained into how magnesium status affects susceptibility to physiological disorders, such as cardiomyopathy and cancer, that are exacerbated by inflammation and by the chemical mediators of anaphylaxis. More information is needed about the impact of less severe magnesium deficiency and of supplemental magnesium on indicators of immune function. Future studies should explore interactive relationships between Mg and such nutrients as vitamin D to elucidate more completely the roles that Mg can play in optimizing immune function.