Magnesium is an important cofactor in hundreds of physiologic processes. Magnesium deficiency is common in hospitalized patients, manifesting commonly as hypokalemia or hypocalcemia, and affects cardiac and neuromuscular function. Hypomagnesemia has been associated with increased mortality in critically ill patients. Magnesium homeostasis is maintained by intestinal absorption and urinary losses of magnesium. Patients with diarrhea, malabsorption, and diuresis are at increased risk for hypomagnesemia. Drugs such as diuretics, proton pump inhibitors, and chemotherapeutic agents are associated with hypomagnesemia. Intravenous magnesium may be necessary to correct hypomagnesemia. Because magnesium is primarily intracellular, the serum magnesium may not accurately reflect the total body stores of magnesium.