Behind New York and Los Angeles, the city of Chicago, formally the City of Chicago, is the most populous city in the United States of Illinois and the third most populous city in the country, after those of New York and Los Angeles. It is also the most populated city in the Midwestern United States, and the sixth most populous city in North America, according to the 2020 census. A tiny part of Chicago's 'Hare International Airport extends into DuPage County, which is the county seat of Cook County, the second most populous county in the United States. Chicago is also the county seat of the second most densely populated county in the United States. Illinois' largest city, Chicago is the main city of the Chicago metropolitan region, which may be classified as either the metropolitan statistical area (9.6 million people) or the combined statistical area (nearly 10 million inhabitants), which is referred to as Chicagoland by locals. It is one of the world's 40 most populous metropolitan regions.
Chicago, which is situated on the beaches of freshwater Lake Michigan, was established as a city in 1837 at a portage between the Great Lakes and the Mississippi River basin, and it developed quickly throughout the mid-19th century. Following the Great Chicago Fire of 1871, which burned several square miles of the city and forced more than 100,000 people from their homes, the city rebuilt itself. With an increase in building activity over the following decades, Chicago grew to become the fifth-largest metropolis in the world by 1900, less than 30 years after the Great Fire of 1871. Chicago made significant contributions to urban planning and zoning regulations, including the creation of new building types (including the Chicago School of architecture), the establishment of the City Beautiful Movement, and the invention of the steel-framed skyscraper, among other things.