Submerged membrane bioreactors (SMBRs) operate at constant imposed flux usually with the membranes in direct contact with the mixed liquor. Membrane fouling is evident as a rise in the required transmembrane pressure (TMP). In this paper we describe the rise in TMP as a three stage process. Stage 1 occurs in a period of a few hours and involves abrupt TMP rise due to 'conditioning', presumably by pore blockage and closure. Stage 2 is a prolonged period of slow TMP rise, which we ascribe to accumulation of extracellular polymeric substances (EPS) and other products of bioactivity, either deposited from the bulk liquor or produced in biofilms on the membrane surface. Polysaccharides are predominant in the supernatant EPS, and appear to be major membrane foulants deposited by the filtration process. Stage 3 is a sudden rise in TMP, which rapidly leads to inoperability of the membranes. This stage could have several causes, which we summarize, and which are all driven by the self-accelerating nature of fouling under constant flux operation. In order to understand the complex nature of fouling in submerged MBRs we have developed the MBR 'roadmap' that shows the relationship between the many operational and design characteristics of the MBR and the generic 'fouling factors', i.e. the nature of the feed (to the membrane), the membrane properties, and the hydrodynamic environment. We also present a MBR 'fouling mechanism map' which depicts the three stages of fouling and the multiple fouling mechanisms that could occur. Experimental observations are provided in support of many of the proposed fouling mechanisms.. (c) 2006 Published by Elsevier B.V.