Core Messages: Cardiopulmonary bypass is a significant risk factor for acute kidney injury (AKI) in children with congenital heart disease. AKI is a significant and independent risk factor for increased morbidity and mortality in critically ill patients. The traditional definitions of acute renal failure that rely on changes in serum creatinine alone are no longer valid, as even small increases in serum creatinine are associated with excess morbidity and mortality in critically ill patients. The concentrations of urinary NGAL, IL-18, and KIM-1, and serum NGAL and cystatin C are emerging as novel, early biomarkers of AKI in children following cardiopulmonary bypass. The treatment of AKI is largely supportive, though early recognition is important. Timing of treatment (i.e., renal replacement therapy) may be crucial to assure the best possible outcome.