The artificial membranes used in haemodialysis and haemofiltration are either cellulosic or made of synthetic polymers, such as polyacrylonitrile, polysulfone, polycarbonate, polymethylmetacrylate and ethyvinylalcohol. During dialysis the water and solute transfer primarily depends on hydraulic permeability and sieving coefficients. At present, high-flux membranes have sieving coefficients for urea (Mol. wt 60) to inulin (Mol. wt 5,200) that are similar to those of the glomerular basal lamina, whereas their hydraulic permeability remains well below that of the renal filter. Bioincompatibility factors responsible for acute, anaphylactoid-like reactions and chronic inflammatory complications have been identified. The choice of the correct dialysis membrane must rest not only on performance criteria but also on biocompatibility and economic criteria.