Magnesium is the second most abundant intracellular cation in the human body and serves as an obligate cofactor in approximately 300 essential enzymatic cellular reactions. Magnesium is crucial for the generation and use of adenosine triphosphate and is required for oxidative phosphorylation. A wide variety of internal and external stresses have been found to increase a person's requirement for dietary magnesium. Magnesium deficiency may develop during a variety of pathological states, or it may have an iatrogenic basis. Magnesium deficiency complicates the management of hypertension and cardiac arrhythmias, affects the clinical behavior of neuromuscular blocking drugs, and may impair immune function.