Residual renal function (RRF) remains important even after commencement of dialysis. Its role in the adequacy of peritoneal dialysis (PD) is well recognized and is increasingly utilized in incremental PD regimes, but it is also vitally important in hemodialysis (HD) patients, in whom it, as in PD patients, may improve survival. It may allow for a reduction in the duration of HD sessions. It reduces the need for dietary and fluid restrictions in both PD and HD patients. Other contributions include improved middle molecule clearance, better hemoglobin, phosphate, potassium, and urate levels, enhanced nutritional status and quality of life scores, and better outcomes in pregnancy. On the negative side, hypoalbuminemia may be prolonged in patients with persistent nephrotic-range proteinuria. Contrary to popular belief, RRF does not necessarily decline rapidly with the initiation of HD. PD may be better than HD in preserving RRF, although this difference may not persist if biocompatible membranes, bicarbonate buffer, and ultrapure water are used. Nocturnal ambulatory peritoneal dialysis (APD) patients may fare worse than continuous ambulatory peritoneal dialysis (CAPD) patients. RRF can be adversely affected by angiotensin-converting enzyme (ACE) inhibitors, nonsteroidal anti-inflammatory drugs (NSAIDs), aminoglycosides, and radiocontrast agents. Diuretics can help maintain fluid balance but not RRF.