The halligen have areas ranging from 7 to 956 ha, and are often former parts of the mainland, separated therefrom by storm tide erosion.

Dwellings and commercial buildings are built upon metre-high, man-made mounds, called Warften in German or Værft in Danish, to guard against storm tides.

The commercially developed halligen of Nordstrandischmoor, Gröde, Oland, Langeneß, and Hooge are surrounded by the protected area, but not an integral part of it.

The smaller halligen, Habel, Südfall, Süderoog, and Norderoog as well as the Hamburger Hallig are parts of the national park.

In the west the German halligen are protected from the open sea by the North Frisian Barrier Island.