SUMMARY -Acute renal failure (ARF) is currently more frequently seen as part of a more complex syndrome defined by sepsis and/or multiple organ failure. Evolution in the field of hemodialysis has led to a parallel development of new systems for continuous renal replacement therapy (CRRT) in critically ill patients. The various CRRT modalities differ in the type of vascular access, application of diffusive or convective clearance (or a combination of both), and location where the replacement fluid enters the circuit. CRRTs have certainly facilitated the management of critically ill patients with ARF combined with cardiovascular instability, severe fluid overload, hypercatabolism, cerebral edema, adult respiratory distress syndrome, lactic acidosis, sepsis or other inflammatory syndromes, crush syndrome, congestive heart failure, and cardiopulmonary bypass. Continuous therapies incorporate several advantages including improved hemodynamic stability, optimal fluid balance, gradual urea removal, elimination of septic mediators, and the possibility of unlimited parenteral nutrition. Major difficulties and unsolved problems of CRRT are the ongoing necessity for continuous anticoagulation, considerable loss of amino acids, vitamins, trace elements, potassium, phosphate, and some drugs as well as immobilization of the patient. The advantages of CRRT should theoretically translate into improved outcomes of critically ill ARF patients, but the superiority of continuous modalities in terms of outcomes is still controversial, despite encouraging results in some clinical trials.