Acute renal failure is common among critically ill patients and carries significant morbidity and mortality. The reported incidence and the attributed morbidity and mortality of acute renal failure vary widely, largely owing to the use of a wide variety of definitions for acute renal failure. Until recently, no consensus existed about how to best define, characterize, and study acute renal failure. This lack of a standard definition has been a major impediment to the progress of clinical and basic research in this field. This review outlines some of the physiologic principles that may help us better understand and define acute renal failure and describes the RIFLE criteria (an acronym comprising Risk, Injury, and Failure; and Loss, and End-stage kidney disease), a recent consensus method of defining and stratifying acute renal failure. Also discussed are many of the challenges and controversies associated with achieving consensus and developing a classification for acute renal dysfunction.