City, seat of Cook county, northern Illinois, U.S. Located on the southwestern tip of Lake Michigan, Chicago became the main transshipment centre for the grain and livestock ofthe Midwest in the 19th century. In the 20th century it remained the leading transportation, commercial, and industrial centre of the north-central United States.

Until the 1830s a minor trading post at a swampy river mouth near the southwestern tipof Lake Michigan, Chicago made use of its strategic location as the interior land and water hub of the expanding United States to become the centre of one of the world's richest industrial and commercial complexes. It is the third most populous city and metropolitan area in the United States. Chicago's achievements are distinctly characteristic of the country as a whole, and its problems are the problems of the modern United States; in a sense it may be-as a series of observers has called it-the typical American city.

In addition to Chicago's standing as a major inland port and railroad hub of the nation, O'Hare International Airport is the world's busiest. By the late 1960s the older, smaller Midway Airport had been pressed into service again to relieve congestion.