The Institute of Marine Research in Norway carried out the research programme Mare cognitum on the Norwegian Sea ecosystem from 1993 to 2001. Drawing upon results from this programme, the book summarises the current knowledge about this ecosystem. In a series of chapters the readers are introduced to the geography, currents and water masses that are the abiotic setting or .scene. of the ecosystem, and to the plankton, fish, squids, whales, seals and seabirds that are the key biological components or .actors. in the ecosystem. The .play. is the ecological drama where the animals strive to find their food whilst avoiding to be eaten themselves. The Norwegian Sea ecosystem contains very large biomasses of zooplankton and pelagic fish stocks. It is characterised by pronounced seasonal cycles in plankton production and large-scale migrations of herring and other animal populations.