Acute renal failure in critically ill patients is a growing clinical problem. Options for renal replacement therapy in these patients use convective and diffusive clearance and may be intermittent, as in classic hemodialysis, or continuous. New ways of delivering dialysis in the intensive care unit, such as sustained low-efficiency dialysis, are also under development. It may be that renal replacement therapy needs to be tailored to the needs of each individual patient. Current and future research studies should provide the answers to many of these questions.