Definitions are never "right" or "wrong". They are simply more or less "useful" for a given purpose. The same is true of the clinical syndrome of acute renal failure (ARF), which is common in the ICU [1, 2]. In many ways, its nature and epidemiology resemble those of other loosely defined ICU syndromes, such as sepsis or ARDS. In this physiological note, however, we wish to focus on how our understanding of renal physiology can be used to guide the definition of ARF.