Arlington County is an urban county located in the Commonwealth of Virginia, in the U.S. , directly across the Potomac River from Washington, DC. Originally part of the District of Columbia, by an act of Congress July 9, 1846, the area south of the Potomac was returned (retroceded) to Virginia effective in 1847.

As of the year 2000, the population is 189,453. Its county seat is the census-designated place of ArlingtonGR6. Strictly speaking, it is inaccurate to refer to a city or town of Arlington. All cities within the state are independent of counties, though normally towns may be incorporated within counties. However, Arlington has no existing incorporated towns, and Virginia law prevents the creation of any new municipality within a county that has a population density greater than 1,000 persons per square mile, which Arlington County (and its neighbor, Fairfax County) exceeds.