Virtually all of the artificial membranes such as reverse-osmosis membrane, dialysis membrane, ultrafiltration membrane, microfiltration membrane and gas separation membrane in use industrially also have therapeutic applications. Dialysis membranes used clinically in the treatment of patients with renal failure account for by far the largest volume of membranes and, worldwide, their consumption has reached some 70 million square meters a year. Almost all dialyzers now in use are of the hollow-fiber type. A hollow-fiber dialyzer contains a bundle of approximately 10,000 hollow fibers having an inner diameter of about 200 mu m when wet, a membrane thickness of 20-45 mu m when wet, and a length of 160-250 mm. The walls of the hollow fibers function as the dialysis membrane and the materials used for dialysis membranes are many and varied, and include cellulose-based materials and synthetic polymers. This paper reviews blood purification and dialysis membranes and also discusses differences in performance between the natural kidney and filters to clarify separation mechanism of glomerular basement membranes and filtration membranes. An intelligent membrance is proposed of temperature-responsive dialysis membrane with pores the size of which varies with temperature.