The adult human body holds about 99% of all calcium and about 60% of magnesium in the skeleton. The remaining 1% of total body calcium and 40% of total body magnesium are found in the soft tissues and play important roles in such vital functions as nerve conduction, muscle contraction, energy metabolism, blood clotting, membrane permeability, and hormonal signaling. Blood calcium levels are carefully maintained within very narrow limits by the interplay of several hormones, including 1,25-dihydroxy-cholecalciferol, which control calcium absorption and excretion, as well as bone metabolism. The intracellular levels of magnesium are also very tightly regulated, since their alterations can have profound effects on cardiac and skeletal muscle physiology. Bone is constantly turning over, a continuous process of formation and resorption. In children and adolescents, the rate of formation of bone mineral predominates over the rate of resorption. In later life, resorption predominates over formation. Therefore, in normal aging, there is a gradual loss of bone. It is generally accepted that obtaining enough dietary calcium, magnesium, and vitamin D throughout life can significantly support optimal bone health. Vitamin D is a key regulatory hormone for calcium and bone metabolism. Adequate vitamin D status is essential for ensuring normal calcium absorption and maintenance of healthy calcium plasma levels. Magnesium absorption is independent of vitamin D status.