Biodiversiteit in de Zuidwestelijke Delta offers an evidence-based, historic reference for nature in the South-western Delta of the Netherlands, in the period before the Delta Works. It describes the trends over the last century in bird populations, sea mammals, anadrome fishes, marine benthos, vascular plants and plant communities, as well as the major underlying processes. It is concluded that hardly any species has disappeared, whereas quite a number of new species have arrived. However, the new-comers are generalists, while indicators of the estuarine environment are under stress. Important changes are: a decline in estuarine habitat for plants and animals, a large decrease of tidal dynamics, a disappearance of gradients between saline and freshwater environment, and a severe loss of connectivity between the different water bodies. The construction polders over time has caused a direct loss of intertidal habitat. For a sustainable future the availability of sediment and estuarine dynamics are prerequisites.