In the past three decades, our understanding of the physiologic functions of vitamin D1 has extended and includes many unexpected non-classic biological actions. Vitamin D receptors (VDR) and 25-hydroxyvitamin D-1αhydroxylase, the enzyme responsible for the production of 1,25(OH)2D3, the active form of vitamin D, are present in many tissues which are not associated with calcium or bone metabolism. Indeed, 1,25(OH)2D regulates cell proliferation, differentiation and apoptosis in many normal and cancer cells. Epidemiologic studies have shown that vitamin D deficiency increases the risk of cancer, cardiovascular disease and autoimmune disease. However, large scale randomized controlled intervention trials are needed to prove the effects of vitamin D on the prevention and treatment of these diseases. Expression of vitamin D or its metabolites without a subscript refer to both vitamin D2 and vitamin D3 or their metabolites.