According to C. Michael Hogan, the Wadden Sea is one of the world's seas whose coastline has been most modified by humans, via systems of dikes and causeways on the mainland and low lying coastal islands. The Wadden Sea stretches from Den Helder in the Netherlands in the southwest, past the great river estuaries of Germany to its northern boundary at Skallingen north of Esbjerg in Denmark along a total length of some 500 km and a total area of about 10,000 km². Within the Netherlands it is bounded from the IJsselmeer by the Afsluitdijk.
The islands in the Wadden Sea are called the Wadden Sea Islands or Frisian Islands, named after the Frisians. However, on the westernmost Dutch island, Texel, the Frisian language has not been spoken for centuries. The Danish Wadden Sea Islands have never been inhabited by Frisians. The outlying German island of Heligoland, although ethnically one of the Frisian Islands, is not situated in the Wadden Sea.
The mudflats of the Pilsumer Watt near Greetsiel, Germany
The word wad is Dutch for "mud flat" (Low German and German: Watt, Danish: Vade). The area is typified by extensive tidal mud flats, deeper tidal trenches (tidal creeks) and the islands that are contained within this, a region continually contested by land and sea. The landscape has been formed for a great part by storm tides in the 10th to 14th centuries, overflowing and carrying away former peat land behind the coastal dunes. The present islands are a remnant of the former coastal dunes.
The islands are marked by dunes and wide, sandy beaches towards the North Sea and a low, tidal coast towards the Wadden Sea. The impact of waves and currents, carrying away sediments, is slowly changing the layout of the islands. For example, the islands of Vlieland and Ameland have moved eastwards through the centuries, having lost land on one side and grown on the other.
Although the Wadden Sea is not yet listed as a transboundary Ramsar site, a great part of the Wadden Sea is protected in cooperation of all three countries. The governments of the Netherlands, Denmark and Germany have been working together since 1978 on the protection and conservation of the Wadden Sea. Co-operation covers management, monitoring and research, as well as political matters. Furthermore, in 1982, a Joint Declaration on the Protection of the Wadden Sea was agreed upon to co-ordinate activities and measures for the protection of the Wadden Sea. In 1997, a Trilateral Wadden Sea Plan was adopted.
In June 2009, the Wadden Sea (comprising the Dutch Wadden Sea Conservation Area and the German Wadden Sea National Parks of Lower Saxony and Schleswig-Holstein) was placed on the World Heritage list by UNESCO.