The production of a concentrated urine is achieved by countercurrent multiplication in the renal medulla. While the single effect in the outer medulla is known to be active NaCl reabsorption in the thick ascending limb, the single effect in the inner medulla is not definitively established. However, the passive model of Kokko and Rector [1] and Stephenson [2] remains the most widely accepted mechanism for the single effect in the inner medulla. Continued experimental studies of transport in perfused inner medullary nephron segments and mathematical simulations that incorporate these new experimental values and anatomic complexity will be needed to fully elucidate the process of urinary concentration. In addition, the availability of molecular reagents will permit investigation into the molecular mechanisms that regulate transport proteins which play crucial roles in the urinary concentrating mechanism.