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. The county has 5,294,664 residents, which is 43.3% of all Illinois residents. Cook County's population is larger than that of 29 individual U.S. states and the combined populations of the seven smallest US states. There are over 130 incorporated municipalities in Cook County, the largest of which is the county seat, Chicago, which makes up approximately 54% of the population of the county. That part of the county which lies outside of the Chicago city limits is divided into 30 different townships. Geographically the county is the fifth largest in Illinois by land area and shares the state's Lake Michigan shoreline with Lake County.
Cook County was created on January 15, 1831 out of Putnam County by an act of the Illinois State Legislature. It was the 54th county established in Illinois and was named after Daniel Cook, one of the earliest and youngest statesmen in Illinois history, who served as the second U.S. Representative from Illinois and the first Attorney General of the State of Illinois. Shortly thereafter, in 1839, DuPage County was carved out of Cook County.