Arlington County is a county 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.

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 has no other incorporated towns within its borders. Given these unique characteristics, for statistical purposes the county is included as a municipality within the Washington Metropolitan Area by the United States Census Bureau. As of 2010, Arlington County had a population of 207,627 residents.