Sylt (zĭlt, Ger. zült) [key], island (1994 pop. 5,543), 36 sq mi (93 sq km), Schleswig-Holstein, N Germany, in the North Sea. It is the largest of the North Frisian Islands and is connected by a causeway to the mainland; the two main towns are Westerland, a popular seaside tourist spot, and Kampen, a wealthier resort. Except in its cultivated western tip, the island is covered by dunes and heathland; fishing, agriculture, and tourism are the chief industries. Fortified in the 1930s, Sylt was one an early air target of the Allies during World War II. Erosion is an ongoing problem.