This largely nationalist constituency in the primarily rural south-eastern corner of Northern Ireland includes the Mourne mountains plus fishing and ferry ports.
South Down takes in most of Down District Council, the eastern part of Newry and Mourne District Council, and a small part of Banbridge District Council.
In 2007, the SDLP held two Assembly seats, as did Sinn Féin, with the other two held by the UUP and DUP.
The constituency lost the Ballynahinch area to Strangford in the 2009 changes, potentially putting a Unionist seat at risk here.