Urine output has been scant or absent for 24 hours. One hundred percent of healthy premature, full-term, and post-term infants void by 24 hours of age. Oliguria is defined as urine output <1.0 mL/kg/h for 24 hours. Anuria is defined as absence of urine output usually by 48 hours of age. Oliguria is one of the clinical hallmarks of renal failure. Decreased urine output can be from mild dehydration or acute renal failure (ARF) or acute kidney injury (AKI). ARF/AKI is an acute renal dysfunction and occurs when there is a decrease in glomerular filtration rate, an increase in creatinine and nitrogenous waste products with the loss of ability to regulate fluid and electrolytes. Incidence of neonatal ARF/AKI is around 6–24%. There is a high percentage of ARF/AKI in very low birthweight infants, infants postcongenital heart surgery, infants on extracorporeal membrane oxygenation/extracorporeal life support (ECMO/ECLS) (especially with a congenital diaphragmatic hernia), and infants with perinatal depression.