what county is chicago, il in
Cook County, Illinois. Cook County is a county in the U.S. state of Illinois. It is the second-most populous county in the United States after Los Angeles County, California. As of 2015, the population was 5,238,216. Its county seat is Chicago, the largest city in Illinois and third-most populous city in the United States. More than 40% of all residents of Illinois live in Cook County. Cook County's population is larger than that of 28 individual U.S. states, and the combined populations of the seven smallest states.