At birth, GFR and tubular function of neonates is compromised as compared to older children and adults. These functions are even less developed in premature infants. These facts have a direct bearing on drug dosing, fluid and electrolyte administration, and maintenance of acid-base balance in neonates. Although many detailed methods of assessing renal functions have been provided in this article, laboratory and radiologic studies available in most healthcare facilities are often sufficient to provide a clinically relevant data in most patients, including neonates.