Over last half century, the concept of acute renal failure has evolved and with it our estimates of the incidence, prevalence and mortality. Indeed, until very recently no standard definition of acute renal failure was available, and this lack of a common language created confusion and made comparisons all but impossible. In response to the need for a common definition and classification of acute renal failure, the Acute Dialysis Quality Initiative group of experts developed and published a set of consensus criteria for defining and classifying acute renal failure. These criteria which make up acronym 'RIFLE' classify renal dysfunction according to the degree of impairment present: risk (R), injury (I), and failure (F), sustained loss (L) and end-stage kidney disease (E). However, as these criteria were developed, a new concept immerged. Renal dysfunction was no longer only considered significant when it reached the stage of failure, but a spectrum from early risk to long-term failure was recognized and codified. Subsequent studies have validated these criteria in various populations and have shown that relatively mild dysfunction is associated with adverse outcomes. The term acute kidney injury has subsequently been proposed to distinguish this new concept from the older terminology of failure.