Time and again, shipping accidents and major incidents at industrial establishments have demonstrated how quickly serious water pollution can occur, with effects such as fish mortality and other harmful impacts on the aquatic habitat. In order to minimise the consequences of such incidents, continuous water monitoring is indispensable in the interests of early identification and timely counter-measures. This is all the more essential in an industrial conurbation like Hamburg. Here the water quality measuring network, with a current total of ten measuring stations, has been operating on all important bodies of water since 1988. In addition to averting danger, continuous water monitoring makes a contribution to prevention (detection of illegal discharges) and to observing short-term and long-term changes in water quality.