The most important effect of vitamin D is on bone health and calcium homeostasis. Moreover, vitamin D, functioning as hormone, plays a key role in preventing cancers such as colon CA, prostate CA, breast CA and autoimmune diseases such as multiple sclerosis, type 1 diabetes, Crohn' disease and infectious diseases such as tuberculosis. Deficiency of vitamin D is defined as the level of 25(OH)D being less than 20 ng/ml. It causes bone demineralization leading to rachitism in children and osteomalasia in adults. In this review the metabolism of vitamin D, it's functions, levels, measuring methods, causes of insufficiency, clinical and biochemical evaluation and the relation with cardiovascular diseases, osteoporosis and cancer were evaluated.