A review with 45 refs. concerning the effects and extent of biofilm accumulation in reverse osmosis and ultrafiltration membrane systems used in water purifn. is given. Topics discussed include: effects of biofilm formation in membrane systems (increased sepn. system resistance by the biofilm, the biofilm as a gel phase between water and membrane surface, microbial attack, costs) and extent of biofilm accumulation in membrane systems (biofilm thickness in operating systems, the role of biofilm permeability, the role of nutrients).