borough (district), administrative and historic county of Durham, northeastern England, occupying a limestone plateau generally 300 to 400 feet (90 to 120 metres) in elevation between the Pennine uplands in the west and an inland extension of the North Sea coastal plain in the southeast. Sedgefield was historically associated with the southern limits of both the West Durham Coalfield, with its open-pit mines, and the East Durham Coalfield, where deep shaft mines were opened later. There are no operating coal mines left in Sedgefield.

Aycliffe, also called Newton Aycliffe, was the first official new town in the north of England, designated in 1947 in conjunction with a revamped World War II ordnance factory. The industrial estate established there had expanded by the early 1980s to provide employment for 12,000 people manufacturing various products including axles, gears, and components for radar and television. The nearby light industrial centres of Shildon, Ferryhill, and Spennymoor all are former coal-mining towns.

The thick glacial drift soils support cash-crop farming well suited to this generally urbanized area, and dairy cattle are intensively raised. Limestone quarried in the district is used for road stone and cement and as a fluxing agent for the Teesside iron and steel industries. The town of Sedgefield in the east is known for its horse races and fox hunts. Spennymoor is the administrative centre. Area 84 square miles (217 square km). Pop. (2001) 87,206.