In the last 10-15 years, user-friendly continuous renal replacement therapy (CRRT) machines have played a major role in increasing the popularity of these techniques in intensive care settings. At present it is not clear which modality of renal replacement therapy (RRT) is optimal for critically ill patients with acute kidney injury (AKI). The choice between different modalities should therefore not be based on unproven ''outcome'' advantages but on evaluation of the clinical picture and logistical circumstances. In hypercatabolic patients, CRRT and sustained low-efficiency dialysis (SLED) have been shown to provide similar metabolic control, but uncontrolled studies suggested a better hemodynamic stability during CRRT, intended as a higher mean arterial pressure and/or less frequent need to increase inotropic or vasoactive drugs. The incidence of hemorrhagic complications is higher with CRRT; however, in particular conditions, such as in patients at high risk of bleeding, CRRT can be performed without anticoagulation or with the use of alternative anticoagulation protocols. Among the different modalities, regional anticoagulation with citrate appears to be the most promising, and the continuous development of simplified protocols for citrate CRRT might facilitate the more extensive use of this technique in the near future. The presence of a mismatch between prescribed and delivered dialysis dose is frequently reported as an important drawback of CRRT. However, data from a recent study designed to evaluate the prognostic impact of the intensity of renal support in critically ill patients with AKI showed that the target Kt/V was obtained in only 67-69% of intermittent hemodialysis (IHD) sessions. Data from several studies comparing the costs of different RRT modalities showed that CRRT is more expensive than IHD or SLED. However, the costs related to SLED can fluctuate within a wide range and in particular settings the higher costs of CRRT could be partially justified by logistical advantages. Further improvements in CRRT device characteristics, anticoagulation protocols, and adaptation of dialysis/replacement fluids to clinical needs will possibly contribute to maintaining, in the coming years, the key role of CRRT in the treatment of hemodynamically unstable critically ill patients requiring RRT.