The role of residual renal function (RRF) in the health and quality of life of both pre-dialysis and dialysis patients is equally important and now well established (Termorshuizen, Korevaar et al, 2003). RRF plays an important role in maintaining fluid balance, phosphorus control, and removal of uremic toxins in dialysis patients. The importance of RRF in hemodialysis (HD) patients is less well appreciated and it is believed that RRF declined rapidly in HD patients (Morduchowicz, Winkler et al, 1994; Wang, Woo, et al, 2005). Decline of RRF also contributed significantly to anemia, inflammation, and malnutrition in end-stage renal disease (ESRD) patients (Wang, Sea et al, 2001; Pecoits-Filho, Heimburger et al, 2003; Pecoits-Filho, Heimburger et al, 2002; Wang, Wang et al, 2004). More importantly, RRF has also been shown to be a powerful predictor of mortality, especially in patients on peritoneal dialysis (PD) (Bargman, Thorpe et al, 2001; Brener, Thijssen et al, 2011; Maiorca, Brunori et al, 19951). Glomerular filtration rate (GFR) measured by isotope clearance is considered to be the standard measure of renal function. Other tests, such as serum creatinine, creatinine clearance, urea clearance, an average of the creatinine and urea clearances, and urine volume have been used to assess RRF in chronic kidney disease (Levey, 1990). Despite its limitations, urine volume, the simplest measure of RRF, has been correlated to GFR in studies and most authors defined loss of RRF as urine volume < 200 ml/24 hours (Moist, Port et al, 2000). Urine collections (24 hours for PD, interdialytic for HD) to measure urea and/or creatinine clearance usually done at beginning of chronic dialysis and every 1-3 months in patients with RRF. In this chapter, we will review available data that have shown a positive impact of RRF on the survival and quality of life of dialysis patients, and outline the current strategies to preserve RRF in PD and HD patients.