In addition to being important in calcium and phosphorus metabolism, bone formation, and mineralization, vitamin D also plays a part in the maintenance of immune homeostasis. Vitamin D receptors are expressed in a number of different tissues, including, and perhaps most notably, on immune cells. The presence or absence of activated vitamin D has a number of effects on in vitro immune cell function. This review describes the possible immunoregulatory role of vitamin D in rheumatoid arthritis with clinical and animal studies.