Illinois ( (listen) IL-ə-NOY) is a state in the Midwestern region of the United States. Of the fifty U.S. states, it has the fifth largest gross domestic product (GDP), the sixth largest population, and the 25th largest land area. Chicago is the state's largest city and the fifth largest city in North America with the capital in Springfield, located in the center of the state; other major metropolitan areas include Metro East (of Greater St. Louis), Peoria and Rockford.

With Chicago in the northeast, small industrial cities and immense farmland in the north and center, and natural resources such as coal, timber, and petroleum in the south, Illinois has a highly diverse economy. Owing to its central location and geography, the state is a major transportation hub: the Port of Chicago enjoys access to the Atlantic Ocean through the Great Lakes and Saint Lawrence Seaway, and to the Gulf of Mexico from the Mississippi River via the Illinois Waterway. Additionally, the Mississippi, Ohio, and Wabash rivers form parts of the state's boundaries. Chicago's O'Hare International Airport has been among the world's ten busiest airports for decades. Described as a microcosm of the entire United States, Illinois has long been considered a bellwether in social, cultural, and political terms.What is now Illinois was inhabited for thousands of years by various indigenous cultures, including the advanced civilization centered in the Cahokia region. The French were the first Europeans to arrive, settling near the Mississippi River in the 17th century, in a region they called Illinois Country, part of the sprawling colony of New France. Following U.S. independence in 1783, American settlers began arriving from Kentucky via the Ohio River, and the population grew from south to north. The Louisiana Purchase of 1803 incorporated Illinois into the U.S., and in 1818 it achieved statehood. The Erie Canal brought increased commercial activity in the Great Lakes, and the small town of Chicago became one of the fastest growing settlements in North America, benefiting from its location in one of the few natural harbors on the southern section of Lake Michigan. The invention of the self-scouring steel plow by Illinois transplant John Deere turned the state's rich prairie into some of the world's most productive and valuable farmland, attracting immigrant farmers from Germany and Sweden. In the mid 19th century, the Illinois and Michigan Canal and a sprawling railroad network greatly facilitated trade, commerce, and settlement, making the state a transportation hub for the nation.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. As one of America's most industrialized states, Illinois was an important manufacturing center for much of the 20th century, especially during both world wars. The Great Migration from the South established a large community of African Americans in the state, particularly Chicago, who founded the city's famous jazz and blues cultures. Chicago, which had become one of the country's leading cultural, economic, and population centers, remains a global city; its metropolitan area of Chicagoland encompasses about 65% of the state's population.

Three U.S. presidents have been elected while living in Illinois: Abraham Lincoln, Ulysses S. Grant, and Barack Obama; additionally, Ronald Reagan, whose political career was based in California, was born and raised in the state. Today, Illinois honors Lincoln with its official state slogan Land of Lincoln, which has been displayed on its license plates since 1954. The state is the site of the Abraham Lincoln Presidential Library and Museum in Springfield and the future home of the Barack Obama Presidential Center in Chicago.