The membrane bioreactor (MBR) can no longer be considered as a novel process. This reliable and efficient technology has become a legitimate alternative to conventional activated sludge processes and an option of choice for many domestic and industrial applications. However, membrane fouling and its consequences in terms of plant maintenance and operating costs limit the widespread application of MBRS. To provide a better understanding of the complex fouling mechanisms and propensities occurring in MBR processes, this review compiles and analyses more than 300 publications. This paper also proposes updated definitions of key parameters such as critical and sustainable flux, along with standard methods to determine and measure the different fractions of the biomass. Although there is no clear consensus on the exact phenomena occurring on the membrane interface during activated sludge filtration, many publications indicate that the extracellular polymeric substances (EPS) play a major role during fouling formation. More precisely, the carbohydrate fraction from the soluble microbial product (also called soluble EPS or biomass supernatant) has been often cited as the main factor affecting MBR fouling, although the role of the protein compounds in the fouling formation is still to be clarified. Strategies to limit fouling include manipulating bioreactor conditions, adjusting hydrodynamics and flux and optimizing module design. (c) 2006 Elsevier B.V. All rights reserved.