Acute renal failure (ARF) is a common condition which is a major contributor to morbidity and mortality amongst critical care patients. As renal function is highly dependent upon adequate renal blood flow and glomerular filtration pressure, oliguria is an early marker of an inadequate haemodynamic status. This chapter lists out prerenal causes, intrinsic causes and postrenal causes of renal failure. Early identification of reversible causes of renal impairment with expedient intervention may prevent progression to established ARF. Prerenal uraemia should be treated by urgent restoration of circulating volume and renal perfusion pressure and by correction of electrolyte deficits. Renal replacement therapy (RRT) is the mainstay of treatment for established acute renal failure. Survival following an episode of ARF is variable and, within the critical care environment, is largely dependent upon the accompanying pathological conditions, e.g. severe sepsis.