Ertholmene consists of three main islands, Christiansø (named after King Christian V), Frederiksø (named after King Frederick IV) and Græsholm, plus a number of minor rocks and skerries. The best known of the latter are Tat and Østerskær, Denmark's easternmost point. Christiansø makes up 22.3 hectares, Frederiksø 4, and Græsholm 11 hectares. Only Christiansø and Frederiksø are inhabited, while Græsholm is a bird reserve. The sound between Christiansø and Frederiksø, a well-sheltered natural harbour, is crossed by a pedestrian bridge that is pulled aside to accommodate larger vessels.
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 Denmark needed a naval base in the central Baltic Sea, a fort was built on Christiansø and Frederiksø in 1684 which served as an outpost for the Danish Navy until 1855. Christiansø Church originally served the garrison. The population peaked at the census in 1810 which showed 829 inhabitants. They were soldiers and were there because of the Gunboat War. Many of the historical buildings now serve as living quarters for the local population, and some are rented, year after year, to regular summer residents. The islands' external appearance has changed very little in over 300 years. Girdled by thick granite walls with old cannons pointed seaward, Christiansø is a picturesque tourist spot seemingly frozen in time. A former part of the fort, Store Tårn has housed the Christiansø Lighthouse for the past 200 years, and a small round tower on Frederiksø, Lille Tårn, serves as a museum.
The islands are state property - an unincorporated area - and do not belong to either a municipality or a region, instead, they are governed by an administrator appointed by the Danish ministry of defence which is responsible for carrying out the tasks normally performed by municipalities and other public sector civil services. Major sources of income are fishery and tourism. There are 80,000 tourists per year, mostly day visitors arriving via Bornholm. Ertholmene is also a popular destination for yachts.
The islanders, who do not pay municipal taxes of any kind, never having been part of a municipality, will be exempt from the central government 8% "Health Contribution" tax (Sundhedsbidrag) introduced with Kommunalreformen ("The Municipal Reform" of 2007), because it is anticipated the relatively isolated islets would otherwise be abandoned. This tax replaced the county tax, which the islanders were also exempt from paying, not being part of a county.