Water constitutes approximately 60% of the healthy adult human body. Water balance in the body is regulated by the kidneys, which excrete either concentrated or dilute urine in accordance with physiological needs. This article describes the mechanisms by which the kidneys vary water excretion independent of excretion of other physiological important substrates such as sodium, potassium, hydrogen, and urea. These mechanisms involve the loops of Henle, the distal tubules, the collecting ducts, and the vasa recta, and are under the control of vasopressin (antidiuretic hormone).