Arlington is a principal city in the U.S. state of Texas, located in TarrantCounty. It is part of the Mid-Cities region of the Dallas?Fort Worth?Arlington metropolitan area, approximately 12 miles east of downtown Fort Worth and 20 miles west of downtown Dallas.According to the U.S. Census Bureau's estimate, the city had a population of almost 379,577 at the end of 2013 making it the third-largest municipality in the metropolitan area. Arlington is the forty-eighth-most populous city in the United States, the seventh-most populous city in the state of Texas, and the largest city in the state that is not a county seat.Arlington is home to The University of Texas at Arlington, a major urban research university, the Arlington Assembly plant used by General Motors, the Nuclear Regulatory Commission Region IV, Texas Health Resources, American Mensa, and D. R. Horton.