Sister cities of Arlington County, Virginia

Arlington County is a county and census-designated place in the Commonwealth of Virginia. The land that became Arlington was originally donated by Virginia to the United States government to form part of the new federal capital district. On February 27, 1801, the United States Congress organized the area as a subdivision of the District of Columbia named Alexandria County. In 1846, Congress returned the land donated by Virginia due to issues involving Congressional representation and the abolition of slavery. The General Assembly of Virginia changed the county's name to Arlington in 1920 to avoid confusion with the adjacent City of Alexandria. Arlington County shares with a portion of the independent City of Alexandria (including the former town of Potomac) the distinction of being once in Virginia, then ceded to the U.S. government to form the District of Columbia, and later retroceded to Virginia.

The county is situated in Northern Virginia on the south bank of the Potomac River directly across from Washington, D.C. Arlington is also bordered by Fairfax County and the City of Falls Church to the southwest, and the City of Alexandria to the southeast. With a land area of 26 square miles (67 km²), Arlington is the geographically smallest self-governing county in the United States, and due to state law regarding population density, has no other incorporated towns within its borders. Given these unique characteristics, for statistical purposes the county is included as a central city of the Washington-Arlington-Alexandria MSA by the United States Census Bureau. As of 2012, Arlington County had a population of 220,565 residents. It would be the fourth-largest city in the state if it were incorporated as such.

Due to the county's proximity to Washington, D.C., Arlington is headquarters to many departments and agencies of the federal government of the United States, including the Department of Defense, the United States Drug Enforcement Administration, and the Defense Advanced Research Projects Agency (DARPA). The many federal agencies, government contractors, and service industries contribute to Arlington's stable economy, which has made it one of the highest-income counties in the United States. Arlington is also the location of national memorials and museums, including Arlington National Cemetery, the Pentagon Memorial, the Marine Corps War Memorial, and the United States Air Force Memorial.