St Kilda was declared by UNESCO as a World Heritage Site in 1986 for its natural heritage, and extended in 2004 to include its surrounding marine environment and in 2005 to recognise its importance as a cultural landscape. St Kilda is one of the few World Heritage Sites to hold mixed status for its natural and cultural qualities.
St Kilda is of Outstanding Universal Value for its exceptional natural beauty and significant habitats. It is unique in the very high bird densities that occur in a relatively small area, linked to its range of complex and varied ecological niches. The complex ecological dynamic in the marine zones is essential to the maintenance of both marine and terrestrial biodiversity. The cultural landscape is an outstanding example of land use resulting from a type of subsistence economy based on the products of birds, agriculture and sheep farming and reflecting age-old traditions. The built structures and field systems, the cleits and the traditional stone houses bear testimony to over two millennia of human occupation in extreme conditions.
The National Trust for Scotland owns the archipelago of St Kilda and manages it, in partnership with Scottish Natural Heritage, Historic Scotland, Comhairle nan Eilean Siar (Western Isles Council), Ministry of Defense and its agents QinetiQ.
The Canmore entry for St Kilda is here.