This volume originated in a belief, shared by the two editors, that the time was ripe for a world-wide survey - or at least sampling - of seasonality in freshwater phytoplankton. An opportunity was provided by the International Limnological Congress (S. I. L. ), held at Lyon in August 1983, to plan a one-day symposium on the topic. From this enjoyable and successful occasion, augmented by additional written contributions, the present volume has emerged. As convenors and editors, we are grateful to the contributors for their cooperation in this international venture. The seasonality of phytoplankton is widely conditioned by that of climate. Thus one may expect to find the geographical differentiation of climatic patterns reflected in the seasonal patterns of algal occurrence. Diversity in the global perspective is also introduced by considerations of geomorphology, geochemistry, and genotypically determined response. Nevertheless, the historical base of our subject is firmly rooted in the north-temperate zone. From its fresh waters, and seas, there have evolved virtually all of the approaches and techniques now being applied to the analysis of phytoplankton seasonal dynamics.