This common presentation was given within the Aerobic Granular Sludge Workshop organized during the IWA Biofilm Conference 2011 (Shanghai, China). The aim was to bring the state-of-the-art knowledge on the microbial diversity present in aerobic granular sludge (AGS) biofilms in comparison with the composition of flocculent activated sludge. Three main parts have been discussed. The first part focused on the bacterial ecology and dynamics during the formation of early-stage granules and then of mature granules, after start-up with flocculent activated sludge and under artificial laboratory conditions. The second part focused on the microbial community comparison of flocculent and granular sludge systems operated under real conditions and fed with raw municipal wastewater. The third part consisted in bringing knowledge on the granule formation mechanisms within an aerobic wastewater system designed for phosphorus removal. The presentation served as support for generating a discussion with the microbial ecology, biochemistry and process engineering scientists present on the following topics. From fundamental to application: To which extent is specific AGS microbial ecology knowledge required? Are there more options for microbial population engineering? Is microbial diversity the defining characteristic for granulation or are there universal microbial triggers?