Magnesium, like calcium, is a divalent cation (+2) that is an essential micronutrient. In its active ion form, magnesium plays a key role as cofactor in hundreds of enzymatic reactions, including all reactions involving ATP; it is necessary for regulating electrolyte gradients and action potentials in neurons and muscles; and it modulates various functions of calcium. Magnesium is a critical component of bone that complexes with other nutrients including calcium and phosphorus. Magnesium homeostasis is maintained by a combination of dynamic regulation of intestinal absorption from dietary sources, tight management of renal magnesium excretion and reabsorption in the nephron, and sequestration in/mobilization from bone matrix. Serum magnesium is the most accepted method for measuring magnesium content, with the important caveat that serum magnesium levels often do not reflect local tissue magnesium concentration or total body magnesium content. Many situations result in magnesium deficiency, the signs and symptoms of which overlap with those of abnormalities of calcium and other electrolytes. Hypomagnesemia can be treated with parenteral and enteric magnesium repletion. Hypermagnesemia is much less common and typically is a result of renal disease.