Hirta in the context of World Heritage Sites in Scotland

St Kilda (Scottish Gaelic: Hiort) is a remote archipelago situated 35 nautical miles (65 kilometres) west-northwest of North Uist in the North Atlantic Ocean. It contains the westernmost islands of the Outer Hebrides of Scotland. The largest island is Hirta, whose sea cliffs are the highest in the United Kingdom; three other islands (Dùn, Soay and Boreray) were also used for grazing and seabird hunting. The islands are administratively a part of the Comhairle nan Eilean Siar local authority area.

The origin of the name St Kilda is a matter of conjecture. The islands' human heritage includes unique architectural features from the historic and prehistoric periods, although the earliest written records of island life date from the Late Middle Ages. The medieval village on Hirta was rebuilt in the 19th century, but illnesses brought by increased external contacts through tourism, and the upheaval of the First World War, contributed to the island's evacuation. Permanent habitation on the islands possibly extends back two millennia, the population probably never exceeding 180; its peak was in the late 17th century. The population waxed and waned, eventually dropping to 36 in 1930, when the remaining population was evacuated. Currently, the only year-round residents are military personnel; a variety of conservation workers, volunteers and scientists spend time there in the summer months. The entire archipelago is owned by the National Trust for Scotland.

Dùn is an island in St Kilda, Scotland. It is nearly 1 mile (1.6 kilometres) long. Its name simply means "fort" in Scottish Gaelic (for more information, see "dun"), but the fort itself has been lost – old maps show it on Gob an Dùin (NF109972), which is at the seaward end.

Though Dùn is almost joined to Hirta at Ruiaval, the two islands are separated by Caolas an Dùin (the straits of Dùn). This prevents Soay sheep from Hirta straying onto the island, and thus it is more vegetated.