Vitamin D is essential for a vast number of physiologic processes, and thus adequate levels are necessary for optimal health. During childhood and adolescence, an adequate vitamin D status is needed due to its important role in cell growth, skeletal development and growth. Vitamin D is a fat-soluble vitamin that is naturally found in very few foods, is added to others, and is available as a dietary supplement. It is produced endogenously when ultraviolet light strikes the skin. The adequate intake and status of vitamin D greatly depends on age, concomitant diseases and the use of some medications that are covered in-depth in the article. Recent epidemiologic and experimental evidence has suggested that low vitamin D concentrations seem to be significantly associated with all-cause mortality, cardiovascular disease, cancer (mainly breast, prostate and colorectal), hypertension, metabolic syndrome, diabetes mellitus type 1 and type 2. However, the data supporting protective effects of vitamin D supplementation on conditions other than skeletal diseases like rickets, osteoporosis and osteomalacia are very weak, and the large, controlled clinical trials that are in progress now should resolve this issue.