Map - Cook County, Illinois (Cook County)

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% of all residents of Illinois live within Cook County. As of 2020, the population was 5,275,541. Its county seat is Chicago, the most populous city in Illinois and the third-most-populous city in the United States.

Cook County was incorporated in 1831 and named for Daniel Pope Cook, an early Illinois statesman. It achieved its present boundaries in 1839. Within one hundred years, the county recorded explosive population growth going from a trading post village with a little over 600 residents to four million citizens, rivalling Paris by the Great Depression. During the first half of the 20th century it had the absolute majority of Illinois's population.

There are more than 800 local governmental units and nearly 130 municipalities located wholly or partially within Cook County, the largest of which is Chicago. The city is home to approximately 54% of the entire county's population. The part of the county outside of the Chicago and Evanston city limits is divided into 29 townships; these often divide or share governmental services with local municipalities. Townships within Chicago were abolished in 1902 but are retained for real estate assessment purposes. Evanston Township was formerly coterminous with the City of Evanston but was abolished in 2014. County government is overseen by the Cook County Board, and countywide state government offices include the Circuit Court of Cook County, the Cook County State's Attorney, the Cook County Sheriff, and the Cook County Assessor.

Geographically, the county is the sixth-largest in Illinois by land area and the largest by total area. It shares the state's Lake Michigan shoreline with Lake County. Including its lake area, Cook County has a total area of 1635 sqmi, the largest county in Illinois, of which 945 sqmi is land and 690 sqmi (42.16%) is water. Land-use in Cook County is mostly urban and densely populated. Within Cook County, the State of Illinois took advantage of its Lake Michigan access and the Chicago Portage, beginning with the construction of the Illinois and Michigan Canal in 1848. This helped make the region a central transit hub for the nation. Chicago, with its location on the Great Lakes and via the St. Lawrence Seaway, is a global port city, giving Cook County an international shipping port.

Cook County's population is larger than that of 28 different U.S. states and territories, and larger than the population of 11 of the 13 Canadian provinces and territories. Cook County is included in the Chicago metropolitan Combined Statistical Area (CSA), which has a population of approximately 10 million people.

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. He 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.

The shape of Cook County and the neighboring counties has remained the same since DuPage County was formed. The population in each county and the split of agriculture compared to residential and industrial activity has changed dramatically over the intervening decades to 2020. The county began with 10,201 people in the Census of 1840, growing rapidly to 5,150,233 people estimated for 2019 by the US Census. Growth was rapid in the 19th century, with the County reaching 2.4 million people by 1910. In the 20th century, the County reached 5.1 million population.

Cook County is nearly completely developed, with little agricultural land remaining near the outer county boundaries.