Flow cytometry has been used in aquatic ecology for more than 20 years and has led to remarkable new insights into the aquatic ecosystem, such as the discovery of the oceanic prochlorophytes and the realization of the importance of the tiny picophytoplankton in the open oceans. This volume collects a series of lectures given at a symposium held at the Research and Technology Centre of Kiel University at Büsum, Germany, October 15 and 16, 1998. It represents a comprehensive overview over the state-of-the-art of aquatic flow cytometry at the turn of the century, ranging from the application of new fluorescent probes and stains for taxonomic and physiological investigations, to the development of a novel instrument which allows the in-situ monitoring of phytoplankton.