Vit. D is a steroid prohormone. It is synthesized in the skin under ultra –violet light exposure. Traditionally, Vit.D3, cholecalciferol and calcitriol were known to have a role only in calcium and bone mineral metabolism. This article takes an account of the current view on the impact of Vit D deficiency on human health. Research has now shown the indisputable role of Vit D in prevention/treatment of some cancers, osteoporosis, rheumatoid arthritis, multiple sclerosis, tuberculosis, hypertension, and diabetes mellitus. In addition lower maternal Vit. D levels are associated with Pregnancy Induced Hypertension (PIH), suggesting that Vit. D deficiency may be a modifiable risk factor for PIH. Greater awareness of the problem of a high prevalence of vitamin D inadequacy is required among researchers, clinicians, and patients. Special efforts on the medical and social fronts are necessary to combat this preventable epidemic of vitamin D deficiency.