The KB 36 project on Zooplankton in the Wadden Sea aims to elaborate and set up an operational monitoring of zooplankton in the Wadden Sea. This report provides background information on the ecology of zooplankton in general and knowledge on zooplankton in the Wadden Sea, to identify issues to take into account when setting up monitoring of zooplankton. Although the zooplankton includes many sizes of organisms, the focus is on mesozooplankton (0.2-20 mm). It consists mainly of the abundant copepod crustaceans and larvae of zoobenthic species, and forms a link between the primary production (phytoplankton) and predators such as fish. Some challenges in designing a monitoring programme in the Wadden Sea are related to its high turbidity, shallowness and spatial variability. High turbidity may hamper the application of in situ optical and acoustic methods to detect zooplankton, whereas the shallowness complicate the use of traditional horizontal or oblique net hauls, which are common practise in the open sea. Since the spatial variability in the Wadden Sea is high, with channels connected to the North Sea and shallow tidal flats, the horizonal variation in the composition of zooplankton communities is also likely to be high. Observations should be therefore be made at appropriate temporal, horizontal and vertical resolution to resolve multiscale natural variability. Also the frequency of sampling should be high enough to take account of the (seasonal) life-cycles of zooplankton species, including the larval stage of benthos, especially during the summer half-year.