Much of the progress in the understanding of the physiology of vitamin D and its role in calcium and phosphorus homeostasis has been made in recent years. Active vitamin D plays a central role in the regulation of calcium by increasing calcium and phosphorus absorption in the gastrointestinal tract, by stimulating osteoclast activity and stimulating calcium absorption in the distal tubule. Progressive kidney disease is associated with a decline in production and impairment of the action of vitamin D, which leads to an inadequate serum calcium control. This book clarifies the components in Vitamin D therapy and other health factors in regard to kidney disease.