Management of critically ill patients regularly involves the treatment of water and electrolyte disturbances. Moreover, critical care itself may contribute to volume overload and electrolyte abnormalities. Initial therapy should be followed by consequent diagnostic evaluation. The shift of volume and potassium in severe pancreatitis, for example, may lead to a life-threatening situation. In brain-dead patients, successful organ donation is facilitated by careful maintenance of water and electrolyte homeostasis.