Cook County is a county located in the U.S. state of Illinois. As of 2006, the population was 5,288,655, making it the second largest county by population in the United States (after Los Angeles County, California), larger than the population of 29 states, and accounting for 43.3% of the state's population. The county seat is Chicago, the principal city of its metropolitan area; Chicago makes up about 54% of the population of the county, the rest being provided by various suburbs. Cook County is the 19th largest government in the United States. Cook County has by far more Democratic Party members than any other Illinois county, and is one of the most Democratic counties in the United States. It has only voted once for a Republican candidate in a Presidential election in the last forty years, in 1972, when county voters preferred Richard Nixon to George McGovern by 53.4% to 46%.

Cook County's current County Board president is Todd Stroger.