Peter C. Agre, an American Society of Nephrology member, is the recipient of the 2003 Nobel Prize in Chemistry for his discovery of the aquaporin water channels. The function of many cells requires that water move rapidly into and out of them. There was only indirect evidence that proteinaceous channels provide this vital activity until Agre and colleagues purified aquaporin-1 from human erythrocytes and reported its cDNA sequence. They proved that aquaporin-1 is a specific water channel by cRNA expression studies in Xenopus oocytes and by functional reconstitution of transport activity in liposomes after the incorporation of the purified protein. These findings sparked a veritable explosion of work that affects several long-standing areas of investigation such as the biophysics of water permeation across cell membranes, the structural biology of integral membrane proteins, the physiology of fluid transport in the kidney and other organs, and the pathophysiological basis of inherited and acquired disorders of water balance. Agre's discovery of the first water channel has spurred a revolution in animal and plant physiology and in medicine.