Cook County is a county in the United States state of Illinois. As of the 2010 census, the population was 5,194,675. With more than 40 percent of all Illinois residents, it is the most populous county in Illinois and the second-most populous county in the United States after Los Angeles County, California. Its county seat is Chicago, the third-most populous city in the United States.
Cook County's population is larger than that of 29 individual U.S. states and the combined populations of the seven smallest states. There are 135 incorporated municipalities partially or wholly within Cook County, the largest of which is 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 townships. Geographically the county is the fifth largest in Illinois by land area and shares the state's Lake Michigan shoreline with Lake County. Including its lake area, the county has a total area of , the largest county in Illinois, of which is land and (42.16%) is water. Cook County's land is mainly urban and very densely populated.
Cook County was created on January 15, 1831, out of Putnam County by an act of the Illinois General Assembly. 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 state's first Attorney General. In 1839, DuPage County was carved out of Cook County.