Illinois's 3rd congressional district includes parts of Cook County and DuPage County, and has been represented by Democrat Delia Ramirez since January 3, 2023. The district was previously represented by Marie Newman from 2021 to 2023, Dan Lipinski from 2005 to 2021, and by Lipinski's father Bill from 1983 to 2005.

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.