This report is a contribution to the Strategic Environmental Assessment (SEA4) conducted by the Department of Trade and Industry. The plankton community in the SEA4 area is similar to that in the North Sea, with variations in the abundances of some individual species. Emphasis in this review is put on trends in the abundances of major phytoplankton and zooplankton taxa over the last 40 years. The study is based on a unique long-term dataset of plankton abundance in the North Atlantic and the North Sea acquired by the Continuous Plankton Recorder (CPR). In the phytoplankton community, conspicuous trends are the steady increase in Phytoplankton Colour (an indicator of phytoplankton biomass) since 1960 and the mid 1980s peak in Thalassiosira spp. abundance. Among the zooplankton the decline in the abundance of Calanus finmarchicus and the increase in that of Calanus helgolandicus since 1960 are probably related to increasing sea surface temperatures in the northeast Atlantic. Phytoplankton blooms are discussed, from the normal annual blooms to harmful algal blooms (HABs). Because of the extensive fish farm industry in coastal regions of SEA4, there is considerable interest and ongoing research into HABs, but there is no clear evidence that they are increasing in frequency or intensity due to anthropogenic eutrophication. Other important components of the plankton meroplankton, picoplankton and megaplankton are also reviewed.