In an unselected population, the annual incidence of acute renal failure (ARF) seems close to 200 patients per million inhabitants. In elderly patients, this incidence is five times higher than that of younger patients. Mortality is particularly high in intensive care units and doubles if ARF develops after rather than before admission. Death is mainly due to hypovolemic and septic shock, and to cardiovascular diseases. An increasing number of deaths is related to therapeutic limitation. In many cases, ARF can be prevented, e.g. by correcting any sodium deficit and hypovolemia before a surgical procedure, and by considering the true GFR of a given patient before prescribing a potentially nephrotoxic drug, especially in older patients. A poor previous health status, hospitalization prior to admission, and ARF occurring after admission are important predictive factors of mortality, as well as any acute organ dysfunction. Second generation severity scores seem to have a better performance than older ones. The use of continuous hemodialysis and hemofiltration is increasing in ARF patients, but it is not proven that mortality is thereby reduced. A beneficial effect of biocompatible membranes is not clearly demonstrated in these patients. Later, most ARF patients recover a normal, or nearly normal, renal function. Recovery is delayed in older patients and in those whose oliguric period is prolonged. Lastly, the high cost of therapy in ARF justifies the use of all currently preventive measures in patients at risk.