The evolution of technology and biomaterials has permitted a parallel development of renal replacement therapies in the acute, critically ill patient. From the original description of continuous arteriovenous hemofiltration, new techniques such as continuous venovenous hemofiltration, hemodiafiltration, and high flux dialysis have been developed and clinically used. A parallel improvement in efficiency has been achieved with urea daily clearances as high as 50 L or more. The use of special highly permeable dialyzers has also permitted an increase in the clearances of larger solutes, thus leading to significant removals of chemical substances involved in acute inflammation and sepsis. In this field, recent observations have suggested using hemofiltration with high volumes of fluid exchange. The hardware and software of the newer CRRT systems are key in achieving these results and in safely performing such challenging techniques.