Hyponatremia is one of the most frequent ion and water disorders. It is generally due to disproportionate renal water retention. Severe hyponatremia (<125 mEq/L) is associated with well-known clinical symptoms and manifestations. However, even mild reductions in sodium blood levels have been shown to be associated with increased mortality and with the risk of falls and fractures. The diagnosis of hyponatremia, although requiring simple clinical and laboratory tests, may be complex and difficult. Treatment should be tailored to the clinical manifestations and severity of hyponatremia. Vaptans may be used in euvolemic or hypervolemic hyponatremia and particularly in SIADH.