Vitamin D is a liposoluble prohormon and a secosteroid which is produced in the skin after exposure to sun-shine. It is turned to calsitriol which has an important role in the calcium and phosphate metabolism by different metabolic pathways. Deficiency of vitamin D is related to some factors such as less sunlight exposure, inadequately intake of vitamin D and absorbtion problems. Studies recently done have shown vitamin D deficiency exists in many people. The effects of vitamin D deficiency on numerous diseases and functional disorders such as chronic musculoskeletal pain, Type 1 and Type 2 Diabetes Mellitus (DM), obesity, multiple sclerosis, rheumatoid arthritis, cardiovascular diseases, osteoporosis, microalbuminuria, renal failure involving colon, prostat and breast cancers either are reported in literature.Vitamin D is one of the main structural elements playing an essential role in several metabolic pathways both directly and indirectly providing numerous functions carring vital importance for organism. In spite of causing a lot of functional disorders and diseases, vitamin D replacement therapy in case of deficiency is safe, economic and simple unless neglected. In this review, we aimed to demonstrate the metabolism, functions and effects of vitamin D, increased risks of chronic diseases likely to be encountered in vitamin D deficiency and the importance of vitamin D replacement therapy.