This comprehensive review of the state of our knowledge concerning calcium not only demonstrates its importance to human health, but also defines its many complex roles. The authors summarize the latest basic scientific information on the cellular and metabolic functions of calcium, explaining the leading techniques for accurately measuring its bioavailability, absorption, and kinetics, and showing that calcium deficiency is a major problem while calcium excess a rare one. They also examine the complex regulation of calcium absorption, distribution, and excretion, as well as the multiple interactions of diet/lifestyle, and physical activity in calcium homeostasis. Their discussion of the specific roles of calcium in a variety of clinical disorders-osteoporosis, oral health, obesity, reproductive disorders, and the metabolic syndrome-provides new insights and raises new questions. The complex changes in calcium and phosphate regulation that occur in renal disease and the potential role of calcium in hypertension and vascular disease are also addressed.