Continuous renal replacement therapies (CRRT) are continuous forms of renal functional replacement used to manage acute kidney injury (AKI) in the critically ill patient. Depurative mechanisms include convection, diffusion, and membrane adsorption utilizing high-flux highly permeable biocompatible dialysis membranes. The simultaneous infusion of replacement fluid permits fluid removal without intravascular volume contraction and better hemodynamic stability, metabolic control to almost normal parameters, and removal of large-size toxins and cytokines. Moreover, CRRT allows better long-term clearance of small and middle molecules than other dialysis modalities. This article focuses on the different modalities of CRRT and reviews both the basic concepts and the newest approaches to the management of the critically ill patient with AKI.