Different definitions for acute kidney injury (AKI) once posed an important impediment to research. The RIFLE consensus classification was the first universally accepted definition for AKI, and has facilitated a much better understanding of the epidemiology of this condition. The RIFLE classification was adapted by a broad platform of world societies, the Acute Kidney Injury Network group, as the preferred AKI diagnostic and staging system. RIFLE defines three increasing severity stages of AKI. One- to two-thirds of intensive care unit (ICU) patients develop AKI according to these criteria which is associated with worse outcomes such as increased length of ICU stay, costs, and mortality. Over the last decade the incidence of AKI has increased, probably as a consequence that baseline characteristics of ICU patients have changed. Another factor that may explain this is that more patients are treated in clinical settings that are associated with high risk for development of AKI. In addition, there may be genetically predetermined risk profiles for development of AKI such homozygotes for the low activity form of the COMT gene. Mortality of AKI patients has decreased over the last few decades, especially when underlying severity of illness is considered. An important consequence of this is the increasing number of surviving AKI patients who develop chronic kidney disease and end-stage kidney disease. In the specific setting of cardiac surgery, AKI occurs in 19-45% of patients. Renal replacement therapy is necessary in approximately 2% of this cohort. AKI that occurs within a 7-day period after cardiac surgery is related to perioperative risk factors, such as preexisting chronic kidney disease, acute ischemia, aorta cross-clamping, or use of cardiopulmonary bypass. AKI that occurs after the first week is mostly a consequence of sepsis or heart failure.