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, 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.
Given 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 Pentagon, 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.
information courtesy of wikipedia.com