Imaging techniques, especially ultrasonography and Doppler, can give an effective assistance in the differential diagnosis of acute renal failure (ARF). An resistance Index (RI) value >0.75 is reported as optimal in attempting differential diagnosis between acute tubular necrosis (ANT) and prerenal ARF. In hepatorenal syndrome (HRS) RIs is very increased. In some renal vasculitis, as nodose panarteritis (PN), hemolytic-uremic syndrome (HUS), thrombotic thrombocytopenic purpura (TTP), parenchymal perfusion is reduced and RI increased. In lupus nephritis the RI values are correlated with creatinine level and normal RI are considered as a good prognostic tool. In acute primitive or secondary glomerulonephritis (GN), RI value is normal, with diffuse parenchymal hypervascularization. In acute crescentic and proliferative GN and tubulo-interstitial disease, color Doppler (CD) and power Doppler (PD) reveal a decreased renal parenchymal perfusion, which correlates with increased RI values. In acute thrombosis of renal artery, US color Doppler (DUS) reveals either an absence of Doppler signal or a tardus-parvus pulse distal to the vascular obstruction. In this situation it is possible to visualize hyperthropic perforating vessels that redirect their flow from the capsular plexus to the renal parenchyma. In acute thrombosis of the renal vein Doppler analysis of parenchymal vessels reveals remarkable RI values, sometimes with reversed diastolic flow. In postrenal ARF an adjunct to the differentiation between obstruction and non obstructive dilatation can be found through RIs. Diagnostic criteria of obstruction as reported by literature are: RI>0.70 in the obstructed kidney and, mostly, a difference in RI between the 2 kidneys >0.06-0.1.