Vitamin D insufficiency affects almost 50% of the population worldwide. The high prevalence of vitamin D insufficiency is an important public health issue and it's insufficiency is an independent risk factor for total mortality in the general population. Vitamin D insufficiency is a common public health problem, very often unrecognized and untreated, associated with rickets, dental caries, and growth retardation in children and osteomalacia, osteopenia, osteoporosis, decreased muscle strength, falls, and increased risk of fracture in adults. Vitamin D insufficiency is associated with indoor lifestyle, sun avoidance strategies, obesity, diabetes mellitus, low HDL cholesterol, older age, and distance from the equator, darker skin, winter season, air pollution, smoking, malabsorption, renal and liver disease, and medication. This book explains the affects of Vitamin D deficiency with relation to different diseases