Vitamin D is well known for its regulatory effects on calcium and phosphate homeostasis and its role in the maintenance of bone integrity. Over the past decade, there have been data from biochemical and molecular genetic studies that point to vitamin D having a much wider role than just maintenance of calcium and phosphate metabolism. Vitamin D and its synthetic analogues have been shown to have anticancer properties as well as to modulate the immune system. Recently, vitamin D deficiency has been reported in critically ill patients [1, 2]. However, it is still unclear how this deficiency affects patient outcomes in intensive care. The focus of this chapter is to examine the role of vitamin D in the body, with discussion of its effects on mineral and bone metabolism as well as its pleiotropic effects and the role it may play in the pathophysiology of critical illness.