For small solutes, membrane resistance is now at a level where fluid boundary layers are significant contributors to the mass transfer coefficient. There has been little effort at technological innovation to decrease boundary layer resistance on the blood side of the membrane. In contrast, several design changes have been introduced to decrease dialysate side resistance. These changes, aimed at improving the flow ditribution of dialysate through the fiber bundle in the response to the observations that the mass transfer coefficient and the membrane surface area increases as the dialysate flow rate increases, include optimization of fiber packing density, use of spacer yarns, fiber undulations, and new flow distributor design. These changes have improved clearance of small solutes. How ever , the increases are small, generally less than 10%-reflecting the dominant role that blood flow rate plays in the determining the clearance of small solutes.