Loudoun County () is located in the northern part of the Commonwealth of Virginia in the United States. In 2019, the population was estimated at 413,538, making it Virginia's third-most populous county. Loudoun County's seat is Leesburg. Loudoun County is part of the Washington–Arlington–Alexandria, DC–VA–MD–WV Metropolitan Statistical Area. As of 2018, Loudoun County had a median household income of $136,268. Since 2008, the county has been ranked first in the U.S. in median household income among jurisdictions with a population of 65,000 or more.

Leesburg is the county seat of Loudoun County, Virginia. It was built in 1740 and is named for the Lee family, early leaders of the town and ancestors of Robert E. Lee. In the War of 1812, it was a refuge for important federal documents evacuated from Washington, DC, and in the Civil War, it changed hands several times. The town is situated at the base of Catoctin Mountain and adjacent to the Potomac River, 33 miles (53 km) northwest of Washington, D.C., for which it has largely become a commuter suburb. Its population was 42,616 as of the 2010 census and an estimated 53,727 in 2019. It is Virginia's largest incorporated town within a county (rather than being an independent city).

Prince William County is located on the Potomac River in the Commonwealth of Virginia in the United States. As of the 2010 census, the population was 402,002, on July 1, 2019, the population was estimated to be 470,335, making it Virginia's second-most populous county. Its county seat is the independent city of Manassas.A part of Northern Virginia, Prince William County is part of the Washington metropolitan area. In 2019 it had the 20th-highest income of any county in the United States.

Fauquier is a county in the Commonwealth of Virginia. As of the 2010 census, the population was 65,203. The county seat is Warrenton.Fauquier County is in Northern Virginia and is a part of the Washington metropolitan area. The county is one of the fastest-growing and highest-income counties in the United States.