Normal Mg metabolism has been reviewed. The most expeditious method of identifying disorders of Mg deficiency or excess is to order a serum Mg determination. In our opinion, routine serum Mg would significantly enhance the clinician's ability to identify disorders of Mg metabolism. Refractory K repletion can be avoided by identifying co-existing hypomagnesemia in hypokalemic patients; as a result, prompting the clinician to institute simultaneous repletion of Mg as well as K. While oral Mg is the preferred route of repletion in critically ill patients, intravenous administration of Mg is recommended. Refractory hypotension, mental obtundation, and respiratory arrest occur with significant hypermagnesemia. Hemodialysis is recommended for treatment of symptomatic hypermagnesemia.