Arlington County is a county in the Commonwealth of Virginia, often referred to simply as Arlington or Arlington, Virginia. The county is situated in Northern Virginia on the southwestern bank of the Potomac River directly across from the District of Columbia, of which it was once a part. The county is coterminous with the U.S. Census Bureau's census-designated place of Arlington. Arlington is considered to be the second-largest "principal city" of the Washington metropolitan area.
In 2019, the county's population was estimated at 236,842, making Arlington the sixth-largest county in Virginia by population; if it were incorporated as a city, Arlington would be the fourth most-populous city in the state. With a land area of 26 square miles (67 km2), Arlington is the geographically smallest self-governing county in the U.S., and by reason of state law regarding population density, it has no incorporated towns within its borders. It is the 5th highest-income county in the U.S. by median family income.
Arlington is home to the Pentagon, Reagan National Airport, and Arlington National Cemetery. In academia, the county contains Marymount University, George Mason University's Antonin Scalia Law School, the administrative offices buildings and graduate programs for the Schar School of Policy and Government and the Jimmy and Rosalynn Carter School for Peace and Conflict Resolution, as well as satellite campuses of the University of Virginia and Virginia Tech. It will soon house Amazon HQ2, the co-headquarters of Amazon.
The area that now constitutes Arlington County was originally part of Fairfax County in the Colony of Virginia.