Twenty-five years ago a surprising process of giving back reclaimed land to the sea — called “de-polderization” — has arisen in Western Europe. In spite of its small expansion, this process brings up fundamental questions concerning nature protection, economical interests, landscape and sea defence, because it has developed after the reclamation of 15 000 km 2 of tidal marshes since the XIth century. This article puts emphasis on the different stakes involved, on the actors implied in this much debated policy and on the engendered social reactions. It ends with a classification distinguishing four types of de-polderization according to their objectives, their actors and their location.