Vitamin D has classically been known as a "bone hormone." But recently, vitamin D has been found to play a role in respect to systemic diseases such as cardiovascular disease, cancer, and periodontal disease. It has been reported that approximately 1 billion people worldwide are either vitamin D deficient or insufficient. This paper will address the various roles that vitamin D plays in respect to systemic diseases, the effects of vitamin D deficiency, and how it is diagnosed and treated.