Cook County is the most populous county in the U.S. state of Illinois and the second-most-populous county in the United States, after Los Angeles County, California. More than 40 percent of all residents of Illinois live within Cook County. As of 2020, the population was 5,275,541. The county seat is Chicago, the most populous city in Illinois and the third most populous city in the United States. The county is at the center of the Chicago metropolitan area. Cook County was incorporated in 1831 and named for Daniel Pope Cook, an early Illinois statesman. It achieved its present boundaries in 1839. Within a century, the county recorded explosive population growth, going from a trading post village with a little over six hundred residents to four million, rivaling Paris by the Great Depression. During the first half of the 20th century it had the absolute majority of Illinois's population.

Country: United States
State: Illinois
Region: Northern Illinois
Metro area: Chicago metropolitan area
Incorporated: January 15, 1831
Named for: Daniel Pope Cook
County seat: Chicago
Type: County commission
Body: Board of Commissioners
President: Toni R. Preckwinkle (D)
Area rank: 6th largest county in Illinois
Population (2020): 5,275,541
Time zone: Central (UTC−6)
Summer (DST): Central (UTC−5)
ZIP Code prefixes: 600xx–608xx
Area codes: 224/847, 312/872, 773/872, 708
FIPS code: 17-031
GNIS feature ID: 1784766
Congressional districts: 1st, 2nd, 3rd, 4th, 5th, 6th, 7th, 8th, 9th, 10th, 11th
Website: www.cookcountyil.gov
Wikidata description: county in Illinois, United States
Wikidata id: Q108418
Wikidata label: Cook County
Wikidata aliases: Array
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