There is approximately 1000-1500 g of calcium in the human body, 98-99% of this being in the form of hydroxyapatite in bone. Serum ionized calcium is tightly regulated through a complex interplay of parthormone (PTH), calcitriol (1.25(OH)2,D), and ionized calcium itself, acting through their receptors mostly in parathyroids, intestine, bone and kidney. In the review role of those factors and organs in calcium homeostasis is shortly discussed.