Book excerpt: The use of models in activated sludge design and operation is increasing, with a similar trend seen in education. Starting with the original IAWPRC Activated Sludge Model no 1 (ASM1) and the subsequent ASM2 and ASM2D, the first generation of activated sludge models have played an important role in practice. With the development of the latest IWA Activated Sludge Model no 3 further progress has been made, and given the concurrent development of new methods for characterization of biomass and wastewater, this is a field of vigorous activity at present. The fifth Kollekolle Seminar brought together many of the world's leading experts on the activated sludge process, who have been working with activated sludge models in practice and research. The aim, as with previous seminars was to present the latest research findings, putting them into the proper perspective. From this high-quality programme 22 papers have been selected and revised to provide the best collection of papers on the state of the art of activated sludge modeling. Papers cover the following topics: modeling developments; wastewater and biomass characterization and parameter identification; modeling in practices.