The interest in the application of nanofiltration as well as reverse osmosis in the production of drinking water from surface waters (fresh water as well as seawater) is continuously increasing. This is especially true since the introduction of ultra- or microfiltration as pretreatment, as this configuration (as determined with classical methods for fouling like SDI and MFI) seems to guarantee an optimum feed water quality. However, many operators experience severe problems with biofouling during the operation of such systems, i.e., a slime layer on the membrane surface increases resistance to convectional flow and decreases flux dramatically. Biofouling can in fact make a membrane treatment process economically nonviable. This paper discusses biofouling of membranes; origination of membrane biofilms; estimation of gel formation by deposition-adsorption of dissolved polysaccharides; and, consequences for pretreatment strategies. Includes 6 references.