Vitamin D, a fat-soluble vitamin, is a critical nutrient for calcium homeostasis and bone metabolism in humans leading to rickets and osteomalacia in severe deficiency states. Vitamin D levels of >20 ng/ml are considered sufficient and recommended daily dietary allowance ranges from 400 to 600 IU from infancy to adulthood. Main source of vitamin D is vitamin D-enriched food intake and dermal synthesis. Nonnutritional forms of rickets include vitamin D-dependent rickets which are rare genetic conditions due to mutation in genes encoding vitamin D activator enzymes and require very high doses of vitamin D supplementation. Vitamin D receptor binds to several hundred genomic loci and low levels of vitamin D (<20 ng/ml) have been found in different extraskeletal disease states. Still, there are no good randomized controlled prospective trials demonstrating role of vitamin D supplementation in prevention or treatment of these conditions including autoimmune conditions, cardiovascular conditions, and cancer, and hence no guidelines currently exist to support vitamin D supplementation in these disease states.