Birth marks a period of sudden, profound structural and functional change in the life of the organism. In preparation for birth, the kidney along with other major organ systems undergoes a programmatic series of maturational changes, which culminate at birth in adaptation to a terrestrial environment. For the successful transition to occur, the kidney bears the prime responsibility to maintain homeostasis and body composition. In utero the placenta rather than the kidney maintains homeostasis of fetal body fluids. Renal physiology in the preterm and term infant is primarily a function of gestational age and postnatal days. Successful transition involves changes in renal blood flow, glomerular filtration rate and tubular functions. Knowledge of this continuum of changes in renal physiology is absolutely essential in managing the newborn and more specifically, the sick preterm infant. The newborn kidney receives between 15-18% of the cardiac output, whereas the fetal kidneys receive only 2-4% of the combined ventricular output during the last trimester of gestation.