Acute oliguria in the critically ill postoperative patient, or in the trauma victim after resuscitation, is a substantial clinical problem. The mortality associated with ARF in these settings remains unacceptably high. Evaluation of the oliguric patient must include thorough monitoring for, and correction of, prerenal and postrenal causes of oliguria. In this sense, diagnosis of ARF is one of exclusion. Differential diagnosis is facilitated by microscopic examination of urine and by biochemical analyses of blood and urine for calculating indices of tubular function (urinary-to-plasma ratios of blood urea nitrogen and creatinine, sodium excretion, and clearances of sodium, creatinine, solute, and water). The early detection of an intrarenal defect, as accomplished by using serial measurements of free water clearance, may allow interruption of the process and prevention of ARF. Preventive measures include optimization of hemodynamic status and the use of osmotic diuretic agents (mannitol) and loop diuretics (furosemide, ethacrynic acid, and bumetanide). Dopamine is useful for increasing both renal blood flow and urine flow and may be useful for preventing ARF, but this is not firmly established. Experimentally, other approaches such as modulating the renin-angiotensin system, prostaglandin system, and cellular calcium fluxes have been attempted, but the clinical applicability of these measures is not established. The best approach to ARF is preventing it by knowing which patients are at high risk, by studiously preventing renal insults, and by aggressively treating early indications of renal malfunction using established therapies.