This is the political home of the youngest Ulster Unionist MP, Jeffrey Donaldson, who was agent to Enoch Powell in the 1980s and is now seen as the rising star of the hardline wing of his party.
His seat - the fastest growing in the province - takes in the area to the south and west of Belfast, and takes its name from the River Lagan, which eventually flows on into Belfast.
It is a mainly residential constituency with some textile and light industry, agriculture and a little tourism.
The main town in the seat is Lisburn, a market town which is home to the Irish linen centre, a cathedral, and Thiepval barracks, the headquarters of the British Army in Northern Ireland.
It is convenient for commuters, with the M1 motorway linking it to Belfast, it has reasonably good rail links, and Belfast International Airport lies north of the constituency.
The seat includes the high security Maze Prison which over the years has housed Northern Ireland's paramilitary prisoners. In October 2000, the last prisoners left the jail, after they were released on licence under the Good Friday Agreement.