Despite more than a decade of worldwide research on membrane fouling in membrane bioreactors (MBRs), many questions still remain unanswered. In the light of the complexity of the system, it is not surprising that researchers jumped to conclusions on observing any correlations at all, many of which now have to be re-examined as more and more contradictory results are being published. This work therefore aims at stepping back and at critically re-evaluating fouling characterisation methods and results. MBR technology and fouling in particular have been reviewed extensively in 2006 by Judd [1] and Le-Clech et al. [2]. Since then, a large number of both fundamental studies on the interacting biological, chemical and physical phenomena as well as full-scale data have been published. With a focus on recent discoveries and emerging innovative fouling mitigation strategies that might lead to more economical and robust MBR operation, this work is therefore also meant as an update and supplement to these previous reviews. (C) 2010 Elsevier B.V. All rights reserved.