Chicago, officially the City of Chicago, is the most populous city in the U.S. state of Illinois and the third-most-populous city in the United States. With an estimated population of 2,705,994 (2018), it is also the most populous city in the Midwest. Chicago is the county seat of Cook County, the second-most-populous county in the US, and portions of the city extend westward into neighboring DuPage County. It is the principal city of the Chicago metropolitan area, often referred to as Chicagoland. At nearly 10 million people, the metropolitan area is the third-most-populous in the nation.