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, following New York City and Los Angeles. Chicago is the county seat of Cook County, the second most populous county in the U.S., while a small portion of the city's O'Hare Airport also extends into DuPage County.

Located on the shores of freshwater Lake Michigan, Chicago was incorporated as a city in 1837 near a portage between the Great Lakes and the Mississippi River watershed and grew rapidly in the mid-19th century. After the Great Chicago Fire of 1871, which destroyed several square miles and left more than 100,000 homeless, the city rebuilt.