Phosphorus is one of the most abundant constituents of all tissues, and disturbances in phosphate homeostasis can affect almost any organ system. Most phosphorus within the body is in bone (600–700 g), whereas much of the remainder is distributed in soft tissue (100–200 g). Less than 1% is in extracellular fluids. As the major intracellular anion, phosphorus plays a critical role in many aspects of cell function. These include (a) serving as a source of the high-energy phosphate in adenosine triphosphate (ATP); (b) providing an essential element of phospholipid in cell membranes; and (c) directly influencing a variety of enzymatic reactions (e.g., glycolysis) and protein functions (e.g., the oxygen-carrying capacity of hemoglobin by regulation of 2,3-diphosphoglycerate [2,3-DPG] synthesis). The plasma contains ˜14 mg/dL phosphorus, of which 8 to 9 mg is lipid phosphorus, a trace is an anion of pyrophosphoric acid, and the remainder is inorganic phosphate (Pi). The Pi is present in the circulation as monohydrogen phosphate, which is divalent, and as dihydrogen phosphate, which is monovalent. At pH 7.4, the relative concentrations of monohydrogen to dihydrogen phosphate are 4:1. Although phosphorus is present as phosphate in biologic fluids, the concentration of elemental phosphorus is measured in routine serum assays and averages 2.5 to 4.5 mg/dL in normal adults. In children, normal levels are higher. The mechanism for this difference is not established but may be related to the higher levels of circulating growth hormone in growing children and the associated increases in the tubular reabsorption of phosphate.