A review with many refs. covers the causes and mechanisms of the fouling of reverse osmosis (RO) membranes in water purifn. systems, including inorg. deposits (scaling), org. mols. adsorption (org. fouling), particulate deposition (colloidal fouling), and microbial adhesion and growth (biofouling), with emphasis on biofouling and interactions between the various types of fouling. General principles for biofouling prevention are also discussed.