Total body water and tonicity is tightly regulated by renal action of antidiuretic hormone (ADH), reninangiotensin-aldosterone system, norepinephrine and by the thirst mechanism. Abnormalities in water balance are manifested as sodium disturbances--hyponatremia and hypernatremia. Hyponatremia ([Na+ < 136 meq/ l]) is a common abnormality in hospitalized patients and is associated with increased morbidity and mortality. A common cause of hyponatremia is impaired renal water excretion either due to low extracellular fluid volume or inappropriate secretion of ADH. Clinical assessment of total body water and urine studies help in determining cause and guiding treatment of hyponatremia. Acute and severe hyponatremia cause neurological symptoms necessitating rapid correction with hypertonic saline. Careful administration and monitoring of serum [Na+] is required to avoid overcorrection and complication of osmotic demyelination. Vasopressin receptor antagonists are being evaluated in management of euvolemic and hypervolemic hyponatremia. Hypematremia ([Na+] > 145 meq/l) is caused by primary water deficit (with or without Na+ loss) and commonly occurs from inadequate access to water or impaired thirst mechanism. Assessment of the clinical circumstances and urine studies help determine the etiology, while management of hypernatremia involves fluid resuscitation and avoiding neurological complications from hypernatremia or its correction. Frequent monitoring of [Na+] is of paramount importance in the treatment of sodium disorders that overcomes the limitations of prediction equations.