Vitamin D is a fat-soluble nutrient that humans obtain through the diet and by synthesis in the skin upon exposure to ultraviolet B. Vitamin D is then converted by the liver to 25-hydroxyvitamin D, its major circulating form. This form is the best indicator of vitamin D nutritional status and is easily measured. Under the influence of parathyroid hormone, the kidney then converts 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D, the biologically active, hormonal form of the nutrient that is important in the metabolism of calcium and phosphorus and is critical in building and maintaining healthy bones. Many cell types outside of the skeletal system, including various cells in the skin, also express the vitamin D receptor. In addition, many cell types convert circulating 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D for local use. This metabolite has been shown to exert potent effects on cellular differentiation, cellular proliferation, and immune regulation. It is theorized that by these mechanisms vitamin D and its analogues are effective treatment options for psoriasis and other skin diseases. Insufficient vitamin D nutritional status has been associated with a host of other diseases, most notably cancer. There is evidence that supplementation with vitamin D reduces the overall incidence of cancer, although current evidence is insufficient to prove a causative effect. Sunscreen use blocks the ability of the skin to photosynthesize vitamin D, although the effect this has on the vitamin D status of the general population is unclear.