Hypocalcemia, a rather common electrolyte abnormality, can constitute a medical emergency when signs and symptoms are present. Clinical manifestations of hypocalcemia are due both to the actual level of the serum calcium concentration and to the rate of its fall. Symptomatic hypocalcemia has a typical presentation of neuromuscular irritability known as tetany. The most serious manifestations of tetany are laryngospasm, seizures, and cardiac arrhythmias. Evaluation of the hypocalcemic patient requires consideration of both the emergent nature of the presentation as well as the differential diagnosis. The causes of hypocalcemia are generally divided into those etiologies associated with absent production of parathyroid hormone (the hypoparathyroid states) and those due to an abnormality of vitamin D metabolism. The vitamin D-deficient states are usually associated with secondary increases in parathyroid hormone, thus providing for a convenient laboratory distinction between these two main categories. Magnesium deficiency is a special cause of hypocalcemia due both to parathyroid hormone and vitamin D dysfunction. When patients have symptomatic hypocalcemia, treatment is indicated. Parenteral therapy with intravenous calcium gluconate can rapidly relieve symptoms and provide time for the underlying cause to be evaluated and treated definitively.