This chapter reviews several aspects of vitamin D biology. Vitamin D, produced endogenously in the skin, upon exposure to ultraviolet light (sunlight), should be metabolized twice to be activated and function as a key regulator of mineral ion homeostasis. Vitamin D, bound to the vitamin D binding protein, DBP, is transported to the liver where the enzyme vitamin D 25-hydroxylase (CYP2R1) adds a hydroxyl group on carbon 25 to produce 25-hydroxyvitamin D (25(OH) D). The vitamin D receptor (VDR) is a member of the family of nuclear hormone receptors. This ligand-activated transcription factor belongs to a subfamily that also includes the triiodothyronine, retinoid-X and retinoic acid receptors. The biological effects of 1,25(OH)2D are mediated by a nuclear receptor, the VDR. The VDR belongs to the subfamily of nuclear hormone receptors, which also includes the retinoic acid receptors, retinoid X receptors, and thyroid hormone receptors. The transcriptional activity of the VDR is modulated by posttranslational modifications as well as by association with nuclear receptor coactivators.