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Vitamin D was discovered as a micronutrient that is essential for normal skeletal development and for maintaining bone integrity. Its importance in bone physiology is most apparent in the deficiency state, in which the lack of vitamin D produces rickets in children and osteomalacia in adults. However, vitamin D is more appropriately classified as a hormone, and it is the vitamin D endocrine system that regulates skeletal homeostasis. In response to hypocalcemia and elevated parathyroid hormone, the kidney synthesizes and releases 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], the bioactive, hormonal form of vitamin D. 1,25(OH)2D3 acts on mineral-regulating target tissues such as intestine, bone, kidney, and parathyroid glands to maintain normal calcium and mineral homeostasis. Its predominant role is to enhance the intestinal absorption of dietary calcium and phosphorus. Thus, vitamin D preserves skeletal calcium by ensuring that adequate absorption of dietary calcium takes place. In addition to this calciotropic role, vitamin D functions in a plethora of cellular actions, perhaps the most fundamental of which is cellular differentiation (1). In skeletal tissue, 1,25(OH)2D3 increases osteoclast number (2) possibly by inducing the differentiation of preosteoclasts into mature boneresorbing cells (3). Vitamin D also acts directly on the osteoblast, in which one well-established effect is stimulating the synthesis of several bone matrix proteins including osteocalcin and osteopontin. Thus vitamin D is thought to preserve and maintain the integrity of the bony tissues via an integrated series of diverse effects.