Among the many impressive feats of the kidney is the ability to conserve Na+. When intake of Na+ through the diet is very low, urine Na+ concentrations can fall to sub-millimolar levels. This enables the preservation of extracellular fluid volume under conditions where dietary Na+ is scarce and extracellular volume is reduced. The thesis of this Perspective is that there remain important gaps in our understanding of this function of the kidney, both with respect to the segments of the renal tubule that minimize Na+ losses, and the transport systems involved.