Abstract : Both the Southern North Sea (Belgian Continental Shelf) and the Western Scheldt estuary are monitored for international purposes : Oslo & Paris Commission (Paris Convention 1992), the Joint Assessment Monitoring Programme (JAMP) and the Regional Task Team II North Sea, and national purposes : passible harmful effects of sand axtraction, dredging,Ö DvZ-Ecological monitoring studies parameters in the water column (salinity, pH, dissolved oxygen, material in suspension, turbidity, volatile hydrocarbons and bacterial analyses) and also sediment (Al, Cu, Pb, Zn, Hg, Cd, water and carboncontents, organic matter, grainsize, silt content and bacterial analyses), fish and epibenthos (Cd, Cr, Cu, Fe, Hg, Pb, Zn) and fish diseases. Evaluation of the quality of the marine environment and the possible harmful effects of pollution by studying sublethal effects of contaminants of, especially, fish. The techniques used include measurements of effects on the phase I and II defence systems, genotoxic effects, PAH metabolites in the bile, the occurence of fish diseases and liver nodules. Chemical analysis of organic compounds in biota, sediment and suspended matter are integrated in the study.