The Sixth International Workshop on the Measurement of Microbial Activities in the Cycling of Matter in Aquatic Environments reflected the state of the art in aquatic microbial ecology and documented the path in this discipline which has been paved and broadened by many scientists within the last two decades. 225 participants from 24 countries and three continents registered for this workshop and turned it more into a regular conference than a workshop. 95 lectures and 89 posters were presented. In general the concept of the Helsingor workshop (l99l) was kept including key note lectures and round table discussion groups but, for the first time, we had to organize sessions in parallel. The scope broadened again and included topics which emerged only in the recent past like UV-light effects on dissolved organic matter and microbes and in particular the application of molecular biological techniques to identify the structural composition of aquatic bacterial assemblages and bacteria-specific virus-host interactions. 30 papers are presented in this issue in 4 chapters, as identification for microbes by novel techniques, Microbial food webs, Bacterioplankton growth and nutrient dynamics and Microbes and their activities in sediments.