In this review I have discussed out current understanding of the vitamin D endocrine system. Vitamin D is made available to the body both by intestinal absorption and by photosynthesis in the skin. To be active, vitamin D must be hydroxylated to 250HD, principally in the liver, and to 1,25(OH)2D and 24,25(OH)2D, principally in the kidney. The best studied target tissues for the vitamin D metabolites are bone, kidney, and intestine. However, the list of additional potential target tissues is expanding and includes muscle, endocrine pancreas, parathyroid gland, pituitary, and skin. Disorders of the vitamin D endocrine system can be categorized into three groups: decreased bioavailability, abnormal metabolism, and aberrant target tissue response. A number of illustrative examples for each category have been discussed. Primary biliary cirrhosis typifies the problem of vitamin D malabsorption and disrupted enterohepatic circulation; chronic renal failure is the most devastating problem of vitamin D metabolism; and vitamin D dependent rickets type II is the best example of aberrant target tissue response. However, certain disorders overlap these distinct categories. Others, such as the nephrotic syndrome, which leads to urinary losses of the vitamin D metabolites (presumably bound to DBP), are not readily categorized. Nevertheless, an understanding of the level at which the vitamin D endocrine system is perturbed by any given disorder provides a rational basis for therapeutic intervention.