Hypercalciuria is a very frequent disorder that is defined by a daily calcium excretion rate in excess of 0.1 mmol/kg. Whatever its mechanism, it always expresses an increased input of calcium in extracellular fluid, from intestine and (or) bone. In few instances, hypercalciuria is secondary to an underlying disease that needs to be identified (primary hyperparathyroidism, cancer, granulomatosis...). However, in most cases, it is a primary (idiopathic) disorder that reveals an abnormal handling of calcium by intestinal and renal tubular epithelia. It is then treated by a restricted dietary supply in sodium and animal proteins, and by the use of thiazide diuretics.