Fluid and electrolyte balance is vital for good health at all ages. Water comprises 75% of the body weight of a neonate. However, during the first 3 to 5 days of life, 5% to 10% of body weight is lost because of a loss of extracellular fluid. Total body fluid decreases gradually to about 60% of total body weight by 1 year of age. The limited ability of the neonatal kidney to excrete and reabsorb water and the impaired ability to excrete sodium compromise fluid and electrolyte homeostasis in the neonate. However, the neonate born at term can conserve sodium through tubular reabsorption. Fluid requirements change dramatically during the early neonatal period, through childhood, and into adult life. Fluid requirements when expressed as a proportion of body weight are greatest during infancy, when the child is most susceptible to fluid losses. Fluid losses from the gastrointestinal tract are the most profound and clinically devastating. Infants and young children are susceptible to intestinal infections, particularly those that cause watery diarrhea. The discovery that oral administration of oral rehydration therapy (ORT), a glucose-electrolyte solution, can rehydrate and maintain hydration in the infant with acute infective diarrhea is one of the greatest scientific advances during the past 50 years.