Continuous renal replacement therapy (CRRT) is the preferred treatment for acute kidney injury (AKI) in intensive care units (ICUs) throughout much of the developed world. Despite its widespread use, however, no formal proof exists that patient outcomes are improved when CRRT is used in preference to intermittent hemodialysis (IHD). In addition, controversy and center-specific practice variation in the clinical application of CRRT continues, owing to a lack of randomized multicenter studies of both CRRT and IHD providing level 1 data to inform clinical practice. Now, however, the publication of results from the Veterans Affairs/National Institutes of Health Acute Renal Failure Trial Network (ATN) study and the Randomized Evaluation of Normal versus Augmented Level Renal Replacement Therapy (RENAL) trial have provided an unparalleled quantity of information to guide clinicians. These pivotal trials investigated different intensities of CRRT in the ICU and provided level 1 evidence that effluent flow rates >25 ml/kg per hour do not improve outcomes in patients in the ICU. In this Review, we discuss the background and results of the ATN and RENAL trials and the emerging consensus that CRRT is the most appropriate treatment for AKI in vasopressor-dependent patients in the ICU. Finally, we describe the remaining controversies regarding the use of CRRT and the questions that remain to be answered.