A review on the interaction of Ca and vitamin D in bone metabolism, prevention and therapy of osteoporosis, promotion of intestinal Ca resorption and mineralization of bone matrix by vitamin D, inhibition of parathormone by vitamin D, and recommendations of Ca intake and vitamin D supplementation. Ca supply, physiol. effects of Ca, vitamin D supply, vitamin D reference range, vitamin D deficiency and secondary hyperparathyroidism, and other physiol. effects of vitamin D are discussed.