SummaryThe distribution of calcium in the human body and the mechanism responsible for extracellular and intracellular calcium homeostasis are reviewed. Structure, secretion and mode of action of parathormone, calcitonine and vitamine D are discussed with respect to human physiology and physiopathology. Finally the role of calcium in stimulus-contraction coupling and stimulus-secretion coupling is discussed. The possible interactions of calcium with the second messenger system and with the prostaglandines are mentioned in this connection.