Cook County is a county in the U.S. state of Illinois, with its county seat in Chicago. It is the second most populous county in the United States after Los Angeles County. The county has 5,194,675 residents, which is 40.5 percent 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 U.S. states. There are over 130 incorporated municipalities in 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. Cook County is mainly urban and very densely populated, containing the City of Chicago and many suburbs.

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. Shortly thereafter, in 1839, DuPage County was carved out of Cook County.

Illinois (ⁱ/ˌɪlɨˈnɔɪ/ IL-i-NOY) is the 25th most extensive and the 5th most populous of the 50 United States, and is often noted as a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal, timber, and petroleum in the south, Illinois has a broad economic base. Illinois is a major transportation hub. The Port of Chicago connects the state to other global ports from the Great Lakes, via the Saint Lawrence Seaway, to the Atlantic Ocean; as well as the Great Lakes to the Mississippi River, via the Illinois River. For decades, O'Hare International Airport has ranked as one of the world's busiest airports. Illinois has long had a reputation as a bellwether both in social and cultural terms and politics.

Although the state's largest population centers today are in northern Illinois, originally the state's population grew from south to north, with settlers arriving from Kentucky in the 1810s. In 1818, Illinois achieved statehood. Chicago was founded in the 1830s on the banks of the Chicago River, one of the few natural harbors on southern Lake Michigan. Railroads and John Deere's invention of the self-scouring steel plow turned Illinois' rich prairie into some of the world's most productive and valuable farmlands, attracting immigrant farmers from Germany and Sweden. By 1900, the growth of industrial jobs in the northern cities and coal mining in the central and southern areas attracted immigrants from Eastern and Southern Europe. Illinois was an important manufacturing center during both world wars. The Great Migration established a large community of African Americans in Chicago that created the city's famous jazz and blues cultures.

Toni Reed Preckwinkle (born March 17, 1947) is the current Cook County Board President and a former alderman in the Chicago City Council representing Chicago's 4th ward in Cook County, Illinois, United States. She was elected on November 2, 2010, as president of the Cook County Board. Preckwinkle first sought office in 1983 and was defeated twice before securing election in 1991 and subsequently being re-elected as alderman four times.

Preckwinkle has been an occasional critic of Chicago Mayor Richard M. Daley. In her first four terms in office she emerged as the council's prominent defender of affordable housing. Among other issues, she is known for her sponsorship of living wage ordinances, her expressed concerns regarding the costs and benefits of the city's Olympic bid, and her strong stance against police brutality and excessive force.

Preckwinkle was born in St. Paul, Minnesota, moving to Chicago to study at the University of Chicago in the Hyde Park community area, where she earned her bachelor's and master's degrees. She is married to Zeus Preckwinkle, a seventh and eighth grade teacher at Ancona Montessori School. They have two children. Her husband is Caucasian, which at times has been a campaign issue.