Arlington County is a county in the U.S. state of 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 coextensive with the U.S. Census Bureau's census-designated place of Arlington. Arlington County is considered to be the second-largest "principal city" of the Washington metropolitan area, although Arlington County does not have the legal designation of independent city or incorporated town under Virginia state law.
In 2020, the county's population was estimated at 238,643, making Arlington the sixth-largest county in Virginia by population and the largest unincorporated community in the United States; if it were incorporated as a city, Arlington would be the third most populous city in the state. With a land area of 26 sqmi, 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, and is the 11th most densely populated county in the United States.
Arlington is home to the Pentagon (headquarters of the U.S. Department of Defense), the Defense Advanced Research Projects Agency (DARPA), Reagan National Airport, and Arlington National Cemetery. In academia, the county contains Marymount University, the satellite campuses and research programs of George Mason University's Antonin Scalia Law School, Schar School of Policy and Government, and the Jimmy and Rosalynn Carter School for Peace and Conflict Resolution, as well as graduate programs, research, and non-traditional student education centers of the University of Virginia and Virginia Tech. Arlington is also the future home of the co-headquarters of the Big Tech company Amazon, and the global headquarters of aerospace manufacturing and defense industry giants Boeing and Raytheon Technologies.
