Arlington County, or simply Arlington, is a county in the U.S. state of Virginia. The county is located in Northern Virginia on the southwestern bank of the Potomac River directly across from Washington, D.C., the national capital. Arlington County is coextensive with the U.S. Census Bureau's census-designated place of Arlington. Arlington County is the eighth-most populous county in the Washington metropolitan area with a population of 238,643 as of the 2020 census. If Arlington County were incorporated as a city, it would rank as the third-most populous city in the state. With a land area of 26 square miles (67 km2), Arlington County is the geographically smallest self-governing county in the nation. Arlington County is home to the Pentagon, the world's second-largest office structure, which houses the headquarters of the U.S. Department of Defense. Other notable locations are DARPA, the Drug Enforcement Administration's headquarters, Reagan National Airport, and Arlington National Cemetery. Colleges and universities in the county include Marymount University and George Mason University's Antonin Scalia Law School, School of Business, the Carter School for Peace and Conflict Resolution, and Schar School of Policy and Government. Graduate programs, research, and non-traditional student education centers affiliated with the University of Virginia and Virginia Tech are also located in the county. Corporations based in the county include the co-headquarters of Amazon, several consulting firms, and the global headquarters of Boeing, Raytheon Technologies and BAE Systems Platforms & Services.