Increasing the dialysis dose has failed to have an impact on the mortality of dialysis patients. In contrast, residual kidney function (RKF) has consistently been a potent predictor of improved survival for both hemodialysis and peritoneal dialysis patients. The benefits of RKF preservation challenge historical concepts of dialysis adequacy that have focused principally on achieving small-solute clearance targets. In this review, we highlight the evidence supporting the impact of RKF on survival and quality of life of dialysis patients and provide a mechanistic framework for this relationship. Current strategies to preserve RKF also are reviewed, emphasizing those used in patients treated with peritoneal dialysis.