One to 25% of critically ill patients who are admitted to intensive care unit (ICU) develop acute renal failure (ARF), depending on the definition used (1). ARF has a significant impact on morbidity and represents an independent risk factor for mortality. The mortality rate for severe ARF exceeded 50% over the past three decades (2–6). The wide range of incidence in the literature depends on the lack of a reliable definition of the syndrome. On the basis of the most recent RIFLE classification (an acronym indicating Risk of renal dysfunction, Injury to the kidney, Failure of kidney function, Loss of kidney function, and ESRD) (7), ARF can be stratified for severity, and different outcomes depend on the degree of severity as assessed by the extent of GFR loss. Current management depends on the level of severity and includes optimization of hemodynamics and fluid status, avoidance of further renal insults, optimization of nutrition, and, when appropriate, the application of renal replacement therapy (RRT). Indications for RRT generally are clear for patients with the most severe of conditions ( e.g. , anuria with severe hyperkalemia in the setting of septic shock), whereas they can be a matter of controversy and require individualized assessment in less severe situations ( e.g. , polyuric ARF in a patient who has previous chronic renal dysfunction and is otherwise well 2 d after cardiac surgery). Optimal strategies to improve patient outcome in ARF may include optimization of delivered RRT dose (8–15). This review focuses on RRT dose and its measurement and prescription in the ICU and on the current evidence concerning the relationship between RRT dose and outcome. The conventional view of RRT dose is that it is a measure of the quantity of blood purification achieved by means of extracorporeal techniques. As this broad …