The development of recent standardized definitions of acute kidney injury (AKI) has allowed us to begin understanding pediatric AKI epidemiology and risk factors and to stratify outcome by AKI severity. AKI incidence will vary with illness severity of the population studied and definition type, ranging from less than 1% when need for dialysis is used to 82% when less conservative definitions (such as > or =1.5 times baseline serum creatinine) are used to define AKI. The most common AKI causes are secondary, such as sepsis, nephrotoxic medication, and ischemia, each leading to acute tubular necrosis (ATN). Children undergoing cardiopulmonary bypass surgery, stem cell transplantation, or with multiple organ dysfunction syndrome are at high risk for these events. A key feature in diagnosis and management includes identifying the presence of ATN versus a reversible hypovolemic state because patients with ATN may quickly develop fluid overload with overaggressive fluid therapy, requiring dialytic removal. Despite advances in acute pediatric dialysis therapy and in overall care of critically ill children, severe AKI still is associated with a high mortality rate, necessitating more research in early AKI identification and therapeutic trials.