Phosphorus is mainly present in the serum as phosphate which is the principal intracellular anion, essential for the organism, in particular through its role in the production of ATP. In plasma, only inorganic phosphorus (Pi) is measured. In this article, we will review new information about the regulation of Pi at the intestinal, osseous and mainly renal levels. The various cotransporters sodium-phosphorus (Na-Pi) will also be examined and the molecular regulation mechanisms approached. Finally, illustration of Pi physiology will be given through various clinical examples.