In human body phosphorus is mostly stored with calcium in bone as hydroxyapatite in dynamic equilibrium with the extracellular fluid compartment. After identification of FGF23 our understanding of phosphate metabolism has profoundly improved. Chronic deregulation of the blood concentration of inorganic phosphate leads to abnormalities in calcification in both skeletal and extraskeletal tissues. Thus, one major purpose to regulate phosphorus is to keep the calcium/phosphorus product in an appropriate range. Parathyroid hormone, vitamin D and FGF23 cooperatively maintain homeostasis of the concentrations, product and net balance of calcium and phosphate. This review focuses on the mechanisms whereby these three hormones interactively regulate phosphate metabolism.