Chicago, Illinois, the largest city in the state and, since 1982, the third most populous in the United States, after New York City and Los Angeles. It is the seat of Cook County. Chicago is located in northeastern Illinois on Lake Michigan, near the Indiana state line.

Advantageously situated in the central part of the country, Chicago is one of the nation's leading commercial and industrial cities and one of its most important transportation hubs. The city is a center of culture and education, with museums, universities, a symphony orchestra, and an opera company that rank among the foremost in the nation. Famous for its architecture, Chicago is generally considered the birthplace of the skyscraper and has many buildings, especially along the lakefront, designed by the great American architects of the 20th century.

The city extends along Lake Michigan for about 25 miles (40 km) and spreads inland up to 12 miles (19 km). It forms the nucleus of a vast urbanized area that includes Cook, Du Page, Kane, Lake, McHenry, and Will counties and stretches into Wisconsin and Indiana.