This paper reviews the long-standing bulking sludge problem in activated sludge systems. Despite the extensive amount of research that has been done on bulking sludge, it still occurs world-wide and a comprehensive solution does not seem to be available. Bulking sludge can be approached as a microbiological problem (occurrence of a specific filamentous bacterium) or as an engineering problem (growth of bacteria with a filamentous morphology). In the first case species-specific solutions should be found, whereas in the latter case, a generic approach might be available. Since bulking sludge is caused by a group of bacteria with a specific morphology, but not a specific physiology we believe that a generic approach would be feasible. Several theories for bulking sludge are discussed. Based on these theories the application and associated problems with the use of biological selectors are critically evaluated. Finally, a set of open research questions is identified.