Vitamin D has long been known for its effect of promoting intestinal absorption of calcium as well as bone mineralization and its deficiency leads to bone disease, rickets, once common in children. Recently, the receptor for vitamin D was found in cell types different from those commonly involved in calcium homeostasis and in vitro studies show that vitamin D is actively involved in cellular differentiation, also playing an antiproliferative, antiangiogenic and immunomodulatory function. In the meantime epidemiological observations showed an association between low levels of vitamin D and certain diseases such as cancer, cardiovascular diseases, diabetes and some immune system defects (allergy, asthma, multiple sclerosis); taken together these data suggests that contrasting deficiency of vitamin D will contribute to lower the risk of several major diseases. Data available in international scientific literature, however, does not allow definitive conclusions about the real benefits of vitamin D supplementation in the general population.