Fairfax County, officially known as the County of Fairfax, is located in the Commonwealth of Virginia in the United States. Located in northeast Virginia, part of the eastern border Fairfax County borders both the City of Alexandria and Arlington County and forms part of the suburban ring around Washington, D.C. The eastern border of Fairfax County also borders the Potamac River and the State of Maryland.

The county is predominantly suburban in character, with some urban and rural areas. Three incorporated towns, Clifton, Herndon, and Vienna, are located entirely within Fairfax County. The independent cities of Falls Church and Fairfax were formed out of areas formerly under the jurisdiction of Fairfax County, but are politically separate, despite the City of Fairfax being the county seat. Nevertheless, the Postal Service has long considered several portions of Fairfax County to be unincorporated Falls Church and Fairfax City. Several portions of the county also have Alexandria mailing addresses; in this case, many locals refer to these neighborhoods collectively as “South Alexandria”, “Lower Alexandria”, or “Alexandria, Fairfax County”. “South Alexandria” communities include Hollin Hills, Franconia, Groveton, Hybla Valley, Huntington, Belle Haven, Mount Vernon, Fort Hunt, Engleside, Burgundy Village, Waynewood, Wilton Woods, Rose Hill, Virginia Hills, Hayfield, and Kingstowne.

Other communities within Fairfax County are unincorporated areas. Virginia law dictates that no unincorporated area of a county may be incorporated as a separate town or city following the adoption of the urban county executive form of government.[77] Fairfax County adopted the urban county executive form of government in 1966.

There has been a proposal to convert the entire county into a single independent city, primarily to gain more control over taxes and roads. The most recent city conversion proposal was made June 30, 2009.

Fairfax County is home to the headquarters of intelligence agencies such as the Central Intelligence Agency, National Geospatial-Intelligence Agency, and National Reconnaissance Office, as well as the National Counterterrorism Center and Office of the Director of National Intelligence.

The government is the largest employer, with Fort Belvoir in southern Fairfax being the county’s single largest location of federal employment. Fort Belvoir is a United States Army installation and a census-designated place (CDP) in Fairfax County, Virginia, and was known as Camp A. A. Humphreys from 1917 to 1935 and Fort Belvoir after 1935.

Fort Belvoir serves as the headquarters for the Defense Logistics Agency, the Defense Acquisition University, the Defense Contract Audit Agency, the Defense Technical Information Center, the United States Army Intelligence and Security Command, the United States Army Military Intelligence Readiness Command, the Missile Defense Agency, the Defense Threat Reduction Agency, and the National Geospatial-Intelligence Agency.

Fort Belvoir is home to the Virginia National Guard’s 29th Infantry Division (Light) and elements of ten Army Major Commands; nineteen different agencies and direct reporting units of the Department of Army; eight elements of the United States Army Reserve and the Army National Guard; and twenty-six Department of Defense agencies. Also located here are the 249th Engineer Battalion (Prime Power), the Military District of Washington’s 12th Aviation Battalion which provides rotary-wing movement to the DoD and Congress, a Marine Corps detachment, a United States Air Force activity, United States Army Audit Agency, and an agency from the Department of the Treasury. In addition, Fort Belvoir is home to National Reconnaissance Office’s (NRO) Aerospace Data Facility-East (ADF-E).

Fort Belvoir was developed on the site of the former Belvoir plantation, seat of the prominent Fairfax family — the namesake of Fairfax County.

Fort Belvoir is home to a number of significant United States military organizations. With nearly twice as many workers as The Pentagon, Fort Belvoir is the largest employer in Fairfax County. Fort Belvoir comprises three geographically distinct areas: main base, Davison Army Airfield, and Fort Belvoir North.

Fairfax County is also home to seven Fortune 500 companies, including three in the Falls Church area; although not located in the independent municipality of Falls Church.