This ICES Symposium, co-sponsored by the Oslo and Paris Commissions (OSPAR) and the Helsinki Commission (HELCOM), was held in Kiel, Germany, from 19 to 21 March 1997 and attended by about 120 scientists from 19 countries. The Conveners, Professor Ted Smayda (USA) and Professor Franciscus Colijn (Germany) were supported by a Steering Committee comprising Professor Berndt Zeitzschel (Germany), Professor Jurgen Lenz (Germany), Dr Chris Reid (United Kingdom), Professor Ulrich Sommer (Germany), and Dr Hein-Rune Skjoldal (Norway). The idea for the Symposium was the result of discussions in the ICES Working Group on Phytoplankton Ecology, when we were regularly being confronted with questions about the natural and man-induced variations in coastal systems. Time and again we were asked whether the impact of anthropogenic nutrient inputs was visible in the biomass, productivity, or species composition of phytoplankton, and more often than not we were unable to give clear answers, simply because the existing data were either ambiguous or lacking. Thus, the idea of compiling the current knowledge on the issue emerged, with the aim of finding out what had to be done to improve our understanding. In a sense, our symposium was therefore comparable to, though more restricted than, the 1995 ‘‘Ar rhus 1975 Revisited’’ Symposium (see ICES Journal of Marine Science, 53 (6)). The growing interest in the use of long-term time series was also clearly shown during the Kiel meeting; possible climatic effects and global change were discussed repeatedly, with the data at best showing significant correlations rather than identifying cause and effect. Another justification for the Symposium was the fact that an increasing number of the scientists responsible for gathering these long-term time series had been retiring from their posts, which was putting the continuation of their series in jeopardy; their data often only partially published, even lost! This topic was also addressed during the Ar rhus Symposium, when considerable apprehension was expressed by colleagues during the preparatory phase. It was foreseen that one of the possible outcomes of the Symposium might be a renewed and growing interest in long-term ecological studies and archiving of valuable monitoring series in easily accessible data banks. A total of 42 papers and 22 posters were presented during six sessions, and 31 appear in this volume. Registered participants attended from the following countries: Argentina (1), Belgium (1), Denmark (5), Estonia (1), Faroe Islands (1), Finland (4), France (2), Germany (35), Greece (1), Iceland (3), Italy (3), Latvia (2), The Netherlands (5), Norway (8), Russia (1), South Africa (1), Spain (5), Sweden (6), the United Kingdom (7), and the USA (7). The Symposium was a great success, both scientifically and socially. On a scientific level, it was shown that variability is an inherent property of planktonic systems, affecting both phytoplankton and zooplankton but also the physical–chemical properties of their habitats. That is to say, variability is just as characteristic a feature of the phytoplankton as is their requirement for nutrients and irradiance for photosynthesis. So, ought not variability be measured on a routine basis to allow the causal factors to be unravelled! An even more pronounced result was the confirmation that long-term variability in plankton and climatic change (reflected for instance in the North Atlantic Oscillation) are closely related. These major scientific results supersede the long-held notion that while variability was a primary feature of fish stocks, in plankton it was trivial and not directly relevant to fisheries. Variability’s close relationship to climate also indicates that plankton time-series analyses are needed if we are fully to understand variability in fish stocks and fisheries, and that such information should be incorporated within the basic methodological and conceptual approaches of biological oceanography. The Symposium revealed a number of long-term data sets and of researchers interested in variability, trends, cycles, and changes in planktonic systems. As well as identifying data sets, presentation of the results provides a much needed collation and synthesis. It is hoped that these proceedings will provide a benchmark by which the integration of such data into contemporary