The syndrome of acute renal failure (ARF) is a common complication of critical illness. Like every other syndrome in the intensive care unit, it requires a consensus definition to progress with a research agenda that is dedicated to preventing and treating it. A consensus definition has been proposed and is being validated. ARF also requires quantification of severity because severity of functional loss is likely to determine the way in which ARF affects outcome. The new consensus definition also offers a quantification of severity. Finally, classification according to pathogenesis would be desirable but remains elusive. Important steps are being taken toward improving the outcome of these patients. Critical care physicians need to understand and participate in this process.