Fundamental to understanding the way in which perturbations in the vitamin D endocrine system can affect human health is an appreciation of the steps involved in the production of the well-recognized active hormonal form, 1,25-dihydroxyvitamin D(3). Thus this paper focuses first on the nature and regulation of the two enzymes responsible for the production of 1,25-dihydroxyvitamin D(3), the 25-hydroxylase in the liver and the 1α-hydroxylase in the kidney. The most important regulators of the 1α-hydroxylase in the kidney are 1,25-dihydroxyvitamin D(3) itself, parathyroid hormone and FGF23. The extent and importance of extra-renal, 1,25-dihydroxyvitamin D(3) synthesis is then considered. Finally the features of the 24R-hydroxylase, which produces 24R,25-dihydroxyvitamin D(3) in the kidney and is induced by and inactivated, 1,25-dihydroxyvitamin D(3)in target cells are described.