Chicago is the largest city in the state of Illinois. With nearly 3 million residents, it is the most populous city in the Midwestern United States and the third most populous city in the USA, after New York City and Los Angeles. Its metropolitan area, commonly named "Chicagoland," is the 27th most populous metropolitan area in the world, home to an estimated 9.7 million people spread across the U.S. states of Illinois, Indiana, and Wisconsin. Chicago is the county seat of Cook County, the second largest county in the United States by population.

Chicago was founded in 1833, near a portage between the Great Lakes and the Mississippi River watershed. Today, the city retains its status as a major hub for industry, telecommunications and infrastructure, with O'Hare International Airport being the second busiest airport in the world. In 2008, the city hosted 45.6 million domestic and overseas visitors. As of 2010, Chicago's metropolitan area has the 4th largest Gross Domestic Product (GDP) of all metropolitan areas in the world.