Renal function is altered in the neonate in comparison to the adult. Although structurally complete in terms of numbers of nephrons by 36 weeks, the newborn kidney is still functionally immature. Renal function undergoes rapid maturation during the first weeks postbirth in both term and preterm infants. During this time there is little margin for error in regulation of fluid and electrolyte balance, especially for the very low-birth-weight infant. Significant risks for all infants include overhydration, dehydration, and electrolyte imbalances. This article describes development of the renal system, anatomic and physiologic characteristics in the neonate, and implications for practice.