Chicago is the largest city in the U.S. state of Illinois, as well as the third-most populous city in the United States with nearly 2.9 million people. It is the seat of Cook County, although a small portion of the northwest tip of the city, near O'Hare Airport lies in DuPage County. Known as the "Second City," the "Windy City," the "City of Big Shoulders," and "Chi-town" (along with other nicknames and colloquial nicknames that reflect the city's character), Chicago is located along the southwestern shore of Lake Michigan. When combined with its suburbs and nine surrounding counties in Illinois, Wisconsin, Indiana, and Southwest Michigan the greater metropolitan area known as Chicagoland encompasses a population of 9.4 million, making it the third-largest in the United States.

Since its 1833 founding as a frontier town of the Old Northwest, Chicago has grown into one of the world's ten most influential world cities. Chicago today is the financial, economic, and cultural capital of the Midwest. The city is recognized as a major transportation, business, and architectural center of the United States. The city's skyscrapers, local cuisine, political traditions, and sports teams are some of its most recognized symbols.

A resident of Chicago is referred to as a Chicagoan. Typically, residents of Chicago will identify themselves with one of the many neighborhoods of Chicago. African Americans form a plurality in the city (just under two-fifths), about one-third are Caucasian, around a quarter Hispanic and one-twentieth Asian, with small amounts of other groups filling in the remainder. Chicago also has several dozen distinct neighborhoods to match its ethnic diversity; the city is divided into 77 community areas, identified in the 1920s by the University of Chicago.