Chicago is the most populous city in the U.S. state of Illinois, and the third-most populous in the United States, following New York City and Los Angeles. With a population of 2,746,388 in the 2020 census, it is the most populous city in the Midwestern United States. Chicago, the county seat of Cook County (the second-most populous U.S. county), is the center of the Chicago metropolitan area, one of the largest in the world.
On the shore of Lake Michigan, Chicago was incorporated as a city in 1837 near a portage between the Great Lakes and the Mississippi River watershed. It grew rapidly in the mid-19th century; by 1860, Chicago was the youngest U.S. city to exceed a population of 100,000. Even after 1871, when the Great Chicago Fire destroyed several square miles and left more than 100,000 homeless, Chicago's population grew to 503,000 by 1880 and then doubled to more than a million within the decade. The construction boom accelerated population growth throughout the following decades, and by 1900, less than 30 years after the fire, Chicago was the fifth-largest city in the world. Chicago made noted contributions to urban planning and zoning standards, including new construction styles (such as Chicago School architecture, the development of the City Beautiful Movement, and the steel-framed skyscraper).