Probably the proper use of water and electrolyte solutions is responsible for saving more lives of seriously ill patients than is the use of any other group of substances. The beneficial effect may be direct, or the patients may be supported until the pathologic process can be corrected by specific measures or surgical intervention. Without a considerable knowledge of the physiology of body water and electrolytes, physicians cannot properly treat diseases accompanied by dehydration, edema, acidosis, alkalosis and shock or plan a rational therapy when fluids must be given parenterally or when electrolytes have been lost in sweat, urine and gastrointestinal secretions. Previous concepts of the physiology of body fluids were dominated by two postulates which are now known to be erroneous. First, cellular membranes were regarded as practically impervious to sodium and potassium, and second, only alterations in extracellular fluids were thought to be readily accessible to fluid therapy.