Chicago is entirely inside Cook County
All 84 ZIP codes with Chicago as their primary USPS place name sit inside Cook County, Illinois (see the full Chicago ZIP code list), and Cook County has been Chicago's home county since the county's creation in 1831 by the Illinois state legislature, the 54th county established in the state. The one asterisk is the city's own municipal boundary, not its ZIP codes: when Chicago annexed O'Hare Airport in 1956, the deal pulled in a small, uninhabited strip on the airport's southwest edge that actually sits in DuPage County. It's a narrower crossing than Atlanta's split between Fulton and DeKalb (no Chicago ZIP code or resident is affected, just airport land), but it means "Chicago never crosses a county line" isn't quite literally true. San Antonio's own county answer turns out to have a comparably tidy exception worth checking before assuming every big Texas city sits in exactly one county the way Houston does.
Cook County is much bigger than just Chicago
Chicago is Cook County's seat and by far its largest city, but the county contains 135 separate incorporated municipalities in total: 23 cities, 1 town, and 111 villages, either entirely or partially within its boundaries. Well-known Chicago suburbs like Evanston, Oak Park, Cicero, and Skokie are all their own separate municipalities inside Cook County, distinct from Chicago itself even though they're commonly lumped together as "Chicago" in casual conversation.
Cook County is the second-largest US county by population, and growing again
Cook County's population of roughly 5.19 million makes it the second-most-populous county in the country, behind only Los Angeles County, California. Cook lost residents through the pandemic years, bottoming out at about 5.13 million in 2022, but it has grown every year since. The Census Bureau's most recent estimate (July 2025) puts the county about 84,000 below its 2020 count, not still falling. Harris County, Texas (Houston's home county) has been the fastest-growing large county in the US, adding roughly 106,000 residents in a single year (2023 to 2024), and it has narrowed the distance to Cook. But Cook still led by about 150,000 people as of the latest count, and whether Harris eventually overtakes it is not yet a settled trend.