Vitamin D from ultraviolet-B (UVB) irradiance, food, and supplements is receiving increased attention lately for its role in maintaining optimal health. Although the calcemic effects of vitamin D have been known for about a century, the non-calcemic effects have been studied intently only during the past two-three decades. The strongest links to the beneficial roles of UVB and vitamin D to date are for bone and muscle conditions and diseases. There is also a preponderance of evidence from a variety of studies that vitamin D reduces the risk of colon cancer, with 1000 IU/day of vitamin D or serum 25-hydroxyvitamin D levels >33 ng/mL (82 nmol/L) associated with a 50% lower incidence of colorectal cancer. There is also reasonable evidence that vitamin D reduces the risk of breast, lung, ovarian, and prostate cancer and non-Hodgkin's lymphoma. There is weaker, primarily ecologic, evidence for the role of vitamin D in reducing the risk of an additional dozen types of cancer. There is reasonably strong ecologic and case-control evidence that vitamin D reduces the risk of autoimmune diseases including such as multiple sclerosis and type 1 diabetes mellitus, and weaker evidence for rheumatoid arthritis, osteoarthritis, type 2 diabetes mellitus, hypertension and stroke. It is noted that mechanisms whereby vitamin D exerts its effect are generally well understood for the various conditions and diseases discussed here.