Abstract: 1. Serum magnesium concentrations may not reflect the state of whole body magnesium stores. 2. In magnesium depletion states, urinary fractional excretion of magnesium should be less than 2% if renal conservation is normal. 3. Hypomagnesemia is a common electrolyte disturbance that is usually mild but can be associated with refractory hypocalcemia and severe neuromuscular and cardiac disturbances. 4. Recently described genetic abnormalities of intestinal and renal magnesium transport are important causes of hypomagnesemia.