Phosphorus is a mineral element essential to cellular function. The prevalence of hypophosphatemia is particularly high in critically ill patients. Hypophosphatemia may be the result of increased renal excretion, reduced digestive absorption, intracellular shift accompanied or not by depletion of the total phosphorus pool, and extreme catabolic states. Clinical manifestations are especially apparent in severe hypophosphatemia and include impairments of the respiratory, musculo-skeletal, pulmonary, and neurological systems. Hypophosphatemia must be corrected, especially if it is severe and symptomatic.