The archipelago consists of the three islands Christiansų, Frederiksų and Gręsholm as well as several rocks and skerries. Only the two main islands are inhabited. The sound between Christiansų and Frederiksų is a well-sheltered natural harbour.
The islands are state property and are governed by an administrator appointed by the Danish government. The permanent population is approximately 100 persons including more than a dozen children. Major sources of income are fishery and tourism. There are 80,000 tourists per year, mostly day visitors from Bornholm. Ertholmene is also a popular destination for yachts.
Fishermen from Bornholm have used Ertholmene for temporary shelter since the middle ages. The first permanent inhabitation was the result of the Danish-Swedish conflicts in the late 17th century. As Danmark needed a naval base in the central Baltic Sea, a fort was built on Christiansų and Frederiksų in 1684 which served as an outpost for the Danish navy until 1855. Although many of the historical buildings now serve as living quarters for the local population, their external appearance has been changed very little to this day.