Changes in urine output and glomerular filtration rate are neither necessary nor sufficient for the diagnosis of renal pathology. Yet no simple alternative for the diagnosis currently exists. Until recently, there has been no consensus as to diagnostic criteria or clinical definition of acute renal failure. Depending on the definition used, acute renal failure has been reported to affect from 1 to 25 of ICU patients and has led to mortality rates from 15 to 60 . The RIFLE criteria were developed to standardize the diagnosis of acute renal failure and in the process the term acute kidney injury (AKI) has been proposed to encompass the entire spectrum of the syndrome from minor changes in renal function to requirement for renal replacement therapy. Thus, AKI is not acute renal failure but a more general description. Small changes in kidney function in hospitalized patients are important and are associated with significant changes in short and possibly long-term outcomes. The RIFLE criteria provide a uniform definition of AKI and have now been validated in numerous studies.