Vitamin D is the collective name for cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2), which are precursors of hormones with an important role in regulation of the metabolism of calcium and phosphates. This review article describes the production of vitamin D3 in the skin by ultraviolet radiation from sunlight, transport of vitamin D and its metabolites in blood, formation of the active hormonal form - calcitriol (1,25-dihydroxyvitamin D) by hydroxylation in the liver and kidney, and termination of the action by catabolism to inactive metabolites.