Diagnosis and classification of acute pathology in the kidney are major clinical problems. Azotemia and oliguria represent not only disease but normal responses of the kidney to extracellular volume depletion or decreased renal blood flow. Changes in urine output and glomerular filtration rate are therefore neither necessary nor sufficient for the diagnosis of renal pathology. However, no simple alternative for the diagnosis currently exists. By examining both glomerular and tubular function, clinicians routinely make inferences not only on the presence of renal dysfunction but also on its cause. However, pure prerenal physiology is unusual in hospitalized patients, and its effects are not necessary benign. Sepsis, the most common condition associated with acute renal failure in the intensive care unit, may alter renal function without any characteristic changes in urine indices, and classification of these abnormalities as prerenal will undoubtedly lead to incorrect management decisions. The clinical syndrome known as acute tubular necrosis does not actually manifest the morphologic changes that the name implies. A precise biochemical definition of acute renal failure has never been proposed, and until recently, there has been no consensus on the diagnostic criteria or clinical definition. Depending on the definition used, acute renal failure has been reported to affect from 1% to 25% of intensive care unit patients and has led to mortality rates ranging from 15% to 60%. From this chaos, two principles emerged: first, the need for a standard definition and, second, the need to classify the severity of the syndrome rather than only consider its most severe form. The RIFLE criteria were developed to achieve these goals, and the term acute kidney injury has been proposed to encompass the entire spectrum of the syndrome, from minor changes in renal function to requirement for renal replacement therapy. Thus, acute kidney injury is not acute tubular necrosis, nor is it renal failure. Small changes in kidney function in hospitalized patients are important and are associated with significant changes in short-term and possibly long-term outcomes. The RIFLE criteria provide a uniform definition of acute kidney injury and have now been validated in numerous studies.