Hemodialysis removes uremic toxins and excess fluid from the blood by diffusion dialysis and ultrafiltration. The efficiency of this process depends on the size, shape, and type of semipermeable membrane used in the hemodialyzer. Although the hollow-fiber kidney remains popular, the parallel-plate hemodialyzer is reestablishing itself, partly because it can use newer types of membranes. With newer membranes, ultrafiltration can be performed without diffusion dialysis, which appears to have important clinical advantages. The dialysate bath used in the hemodialyzer resembles plasma water in composition, with electrolytes added to compensate for abnormalities of end-stage renal disease. Although the technical problems of hemodialysis have been largely overcome, an accurate, easy-to-use method of evaluating the adequacy of the hemodialysis prescription has yet to be devised.