Loudoun County

County in northern Virginia, bordered to the northeast by the Potomac River, to the northwest by the Blue Ridge Mountains, and to the southwest by Bull Run; area 1,349 sq km/521 sq mi; population (1990) 86,100. Its capital is Leesburg. The county, in the Piedmont, has a number of large estates. Horses and cattle are raised, and tobacco, wheat, and corn are grown. There is also some dairying.

Washington Dulles International Airport is in the south, and suburban development is expanding northwest from the Washington area into the county.