The archipelago of the North Frisian Islands and the archipelago of the East Frisian Islands predominate in the North Sea. Helgoland and Neuwerk are also inhabited. The North Frisian Islands are mainland remnants. These were separated from the coast by land subsidence and subsequent flooding.
The East Frisian islands of Borkum, Juist, Norderney, Baltrum, Langeoog, Spiekeroog and Wangerooge are barrier islands (islands made of sand, parallel to the coast), which were formed from sandbanks by the surf dynamics.
The North Frisian islands of Sylt, Föhr and Amrum, on the other hand, are Geestkern islands which consist of a core of Tertiary and Pleistocene strata protruding from the sea, which in surrounded by Holocene sediments. Geestkern islands are remnants that were separated from the mainland by storm surges in the Middle Ages.
The largest German islands in the Baltic Sea are (from west to east) Fehmarn, Poel, Hiddensee, Rügen and Usedom. With the exception of Fehmarn, they are part of a Bodden coast.
Probably the largest and most important islands in inland waters are Reichenau, Mainau and Lindau in Lake Constance and Herrenchiemsee in Lake Chiemsee.