Arlington Countyis acountyin theCommonwealth of Virginia. It is coterminous with theU.S. Census Bureau-census-designated placeof Arlington, which is the second-largestprincipal cityof theWashington metropolitan area.As a result, the county is often referred to in the region simply as "Arlington" or "Arlington, Virginia".
Due to the county's proximity to downtown Washington, D.C., Arlington is headquarters to many departments and agencies of thefederal government of the United States, including theDepartment of Defense(DoD) at thePentagon,Drug Enforcement Administration,Transportation Security Administration(TSA), andDefense Advanced Research Projects Agency(DARPA). It is also home toRonald Reagan Washington National Airport. The many federal agencies, government contractors, and service industries contribute to Arlington's stable economy. Arlington is also the location of national memorials and museums, includingArlington National Cemetery, thePentagon Memorial, theMarine Corps War Memorial, and theUnited States Air Force Memorial.