This chapter will summarize current knowledge in renal replacement technologies in the treatment of critically ill patients with acute kidney injury. The mechanics of different treatment modalities with emphasis on continuous renal replacement therapies will be described, as well as the application of such technologies in the management of fluid overload and metabolic abnormalities. Appropriate composition of dialysate and replacement fluids, use of anticoagulation, choice of vascular access, and potential complications in the use of such technologies will be discussed. Finally, the chapter will compare the main features of the different currently available CRRT machines. © 2013 Springer-Verlag Berlin Heidelberg.