Vitamin D is critical for bone and mineral metabolism. The two major sources of vitamin D in the human body are sun exposure and diet accounting for 80% and 20%, respectively. In the skin, vitamin D synthesis occurs in the Malpighian layer of the epidermis; exposure of 7-dehydrocholestrol to ultraviolet B rays produces previtamin D which is then converted to vitamin D. Vitamin D then undergoes a series of hydroxylations in the liver and kidney, respectively, to generate the active form. The main physiological role of vitamin D is to regulate calcium and phosphorus metabolism and maintain bone health. This constitutes the classical action of vitamin D. In addition, vitamin D also plays some putative role in immune regulation, insulin secretion and glucose metabolism, reproductive function, pituitary hormonal secretion, and other endocrine systems that comprise the nonclassical actions of the hormone. The exact mechanisms of many of these extraskeletal actions are still unclear and need further investigation.