Vitamin D deficiency, which classically manifests as bone disease (either rickets or osteomalacia), is characterized by impaired bone mineralization. More recently, the term vitamin D insufficiency has been used to describe low levels of serum 25-hydroxyvitamin D that may be associated with other disease outcomes. Reliance on a single cutoff value to define vitamin D deficiency or insufficiency is problematic because of the wide individual variability of the functional effects of vitamin D and interaction with calcium intakes. In adults, vitamin D supplementation reduces the risk of fractures and falls. The evidence for other purported beneficial effects of vitamin D is primarily based on observational studies. We selected studies with the strongest level of evidence for clinical decision making related to vitamin D and health outcomes from our personal libraries of the vitamin D literature and from a search of the PubMed database using the term vitamin D in combination with the following terms related to the potential nonskeletal benefits of vitamin D: mortality, cardiovascular, diabetes mellitus, cancer, multiple sclerosis, allergy, asthma, infection, depression, psychiatric, and pain. Conclusive demonstration of these benefits awaits the outcome of controlled clinical trials.