Boreray, Soay, Dun and Hirta are the largest islands in the group.
The name most likely comes from Old Norse sunt kelda meaning sweet wellwater. Other theories invoke a corruption of Hirta or Saint Hilda.
See the article on St Kilda for further discussion on the origin of the name.
The archipelago is a nature reserve recognised by UNESCO and a breeding ground for many important species including puffins and Leach's petrels. The small island of Dun is home to the largest colony of fulmar in Britain.