Vitamin D functions to regulate calcium homeostasis in intestine, kidney, and bone. Vitamin D deficiency during bone development causes rickets and in adults vitamin D deficiency, which has been shown to be common in the elderly population, can cause secondary hyperparathyroidism that can result in osteomalacia and increased risk of fracture. Recent evidence has suggested that vitamin D can have numerous other physiological functions including protection against certain autoimmune diseases, such as diabetes and multiple sclerosis and inhibition of proliferation of a number of malignant cells including breast and prostate cancer cells. Exactly how vitamin D affects numerous different systems is a subject of continuing investigation. This article will review new developments related to the function and regulation of vitamin D target proteins in classic vitamin D target tissues that have provided novel insight into the mechanism of vitamin D action.