The thesis focuses on the nursery function of the Westerschelde and Oosterschelde estuaries for demersal fish and macro-invertebrate species. The main aim of the thesis is to provide a baseline about the higher trophic levels for future process studies in the Delta area. More specifically, both structural and functional patterns in several population parameters (density, biomass, growth, biodiversity, Jood and feeding behaviour) are investigated at community and species levels, and at different temporal (short-, mid- and long-term) and spatial (between and within estuaries) scales. This introductory chapter gives a general description of the epibenthic assemblages and a short overview of the factors on which the nursery function of a system depends. The study area is described and the major engineering works in the Delta area are summarized. An overview of the different sampling methodologies and of the available and used datasets is given. This chapter finishes with the main objectives and an outline of the thesis.