Most of the biological actions of 1,25(OH)2D3, the hormonal form of vitamin D, are mediated by the vitamin D receptor (VDR), a member of the steroid/thyroid receptor superfamily. The VDR functions as a ligand-induced transcription factor regulating the rate of expression of genes involved not only in the control of calcium homeostasis and bone remodeling, but also in immunomodulation, the control of hormone secretion, Inhibition of cell growth, and induction of cell differentiation. This section discusses the molecular mechanisms involved in the transcriptional control of gene expression by 1,25(OH)2D3.