A large body of work in diverse clinical and scientific areas has accumulated that supports a role for vitamin D in multiple organ systems and physiologic and molecular processes. The vitamin D receptor is distributed ubiquitously, and by binding with its receptor, vitamin D initiates a series of events that can affect cellular proliferation and differentiation, inflammation, the immune system, and the endocrine system, including the renin-angiotensin system, insulin resistance, and lipid metabolism. Copyright © 2008 by the American Society of Nephrology.