Among the different classifications of membrane fouling, organic fouling is perhaps the most poorly understood, with relationships to (organic) colloidal fouling and biofouling. Problematical foulants have been identified as polysaccharides and proteins but these components occur in both abiotic macromolecular and colloidal forms. In low-pressure membranes, there is some degree of hydraulic reversibility during backwashing but this option does not exist for high-pressure membranes; in both cases, the effectiveness of chemical cleaning is potentially constrained by the compatibility of membrane material with cleaning agent. Pretreatment by coagulation can reduce organic fouling while there is little consensus on the attributes of powdered activated carbon pretreatment. In general, the organic fouling behavior of membrane bioreactors (MBRs) is consistence with that of low-pressure membranes.