Chicago is a city in the U.S. state of Illinois. It is the third largest city in the United States. As of 2018, the population is 2,705,994. It is the city with the largest population in the Midwestern United States. Chicago is the main city of the Chicago metropolitan area, or Chicagoland. The Chicago metropolitan area has 10 million people. This metropolitan area has the third largest population in the United States.