The development of clinical dialyzers is traced over the past three decades. Beginning with the Kolff rotating device, the evolution of dialyzers has followed a path to reduce size, increase metabolite transport, and minimize blood losses. A combination of design innovations and membrane developments has been necessary to achieve the performance of current dialyzers. These include the development of thin-channel plate dialyzers, hollow-fiber membranes, and more permeable membranes. The development of better devices has been hindered by lack of complete understanding of physiological and biochemical needs for treatment of end stage renal patients. A backlog of engineering and materials information is available for future utilization when therapy needs are clarified. Future trends in membrane developments may include melt-extrudable and "non-isotropic" membranes.