Vitamin D is thought to be important for maintaining normal function of many non skeletal tissues such as muscle (including heart muscle), immune function, and inflammation as well as cell proliferation and differentiation. Studies have shown that it may be useful as adjunctive treatment for tuberculosis, psoriasis, and multiple sclerosis or for the prevention of certain cancers. The consumption of fortified or enriched foods as well as suberythemal sun exposure should be encouraged for people at risk for vitamin D deficiency. The current review focuses on the evidence demonstrating vitamin D deficiency in all age groups.