Phosphorus, or phosphate, is the body’s major intracellular anion involved in numerous biological processes. Most phosphate is intracellular, with the remaining amount contained within soft tissues and the extracellular space. Parathyroid hormone, calcitriol, calcitonin, and phosphatonins regulate normal phosphate homeostasis by adjusting renal and/or gastrointestinal absorption and/or excretion. Hypophosphatemia occurs secondary to decreased gastrointestinal absorption, transcellular shifts, increased renal excretion, or some combination of these general mechanisms. Hyperphosphatemia results from decreased renal excretion, increased intake or iatrogenic administration, transcellular shifts, or some combination of these.