This is a review of article of critically ill renal patients. We discussed severe acute renal failurewhich is seen predominantly in ICU’s.At first, we discussed definitions of acute renal failure with special emphasis on universal one and we reviewed different cases of ARF among ICU patients, which include sepsis, hepatorenal syndrome, rhabdomyolysis, haemolytic uraemic syndrome, post operative cases, contrast nephropathy and drug induced nephropathy. In each case, definition, epidemiology, pathophysiology and treatment were discussed.In the chapter conserning conservative management of critically ill renal patients, we studied different vasoactive drugs and their renal effect. We discussed also the rolw of diuretics, and we studied acid –base disturbances in these patients.At last, we reviewed the renal replacement therapy, its dose, when to start it, how to choose its type and differences between them.