The microbial complexity and the functions of the biomass present in activated sludge and biofilm systems were the subject matter of the 3rd Conference on Microorganisms in Activated Sludge and Biofilm Processes. Detailed knowledge of the identity, physiology and ecology of the microorganisms involved is central to any attempts to modify the composition of the biomass and hence optimise its performance. The Conference covered a wide range of topics dealing with the microbiology and microbial processes in activated sludge and biofilms, including sessions on newly identified microorganisms, microbial selection mechanisms and biomass storage processes. Many of these papers reported on the use of new molecular biological tools that directly address the population structure, dynamics and function of microbial communities. Other sessions dealt with work solving problems and improving performance in full-scale treatment plants and industrial wastewater treatment processes. From the extensive programme, a full peer review has selected for these proceedings 88 of the very best papers on the following topics: microbial dynamics; filamentous bacteria; polyphosphate and glycogen accumulating microorganisms; biofilm composition and processes; nitrifiers and denitrifiers; storage processes in biomass; biomass characterization; microbiology and biochemistry; full-scale experiences; foaming; industrial wastes. Together they form an unrivalled summary of work by the world?s leading experts to research the microbiological basis of wastewater treatment and improve the design and operation of biological processes. These proceedings are an essential reference for engineers, chemists, microbiologists and plant operators researching or applying activated sludge or biofilm techniques.