Birth rapidly changes the demands placed on the kidneys with respect to infant homeostasis. Conceptional age (gestational plus postnatal), general health, and medical management may independently, or, in concert, give rise to important metabolic abnormalities marked by apparent renal functional inadequacies. The chronology of the renal functional changes occurring with maturation in infants born before or at term is now well described. The confounding effects of treatment on the development of renal function in very low birth weight infants are also becoming more apparent. However, the mechanisms responsible for these changes are just becoming to be understood with the use of molecular biologic techniques.