Vitamin D deficiency and insufficiency are global health problems that have significant health consequences. Sunlight remains the major source of vitamin D for children and adults worldwide. Vitamin D requires metabolism in the liver to its major circulating form, 25-hydroxyvitamin D. 25-Hydroxyvitamin D is converted to 1,25-dihydroxyvitamin D in the kidneys. This active hormone is responsible for regulating the efficiency of intestinal calcium absorption and bone calcium mobilization. A wide variety of organs and cells also have the capacity to produce 1,25-dihydroxyvitamin D. The local `production of this hormone plays an important role in regulating a wide variety of genes that control cellular growth and differentiation and production of insulin, parathyroid hormone, renin, and a variety of cytokines, including cathelicidin. A three-step strategy to prevent this worldwide health problem includes sensible sun exposure, ingesting foods containing vitamin D, and taking a vitamin D supplement.