Flag of Illinois	Seal of Illinois

Illinois is the 21st member of the United States and is located in the Midwest region of the United States of America. The state is the most populous in the Midwest, and the fifth most populous in the nation. Illinois is known for its large and diverse population; its balance of rural areas, small industrial cities, vast suburbs and a great metropolis; its highly diverse economic base; and its central location that has made it a transportation hub for 150 years. It is this mixture of factory and farm, of urban and rural that makes Illinois a microcosm of the United States.

About 2,000 Native American hunters inhabited the area at the time of the American Revolution, and a small number of French villagers. American settlers began arriving from Kentucky in the 1810s; they achieved statehood in 1818. Yankees arrived a little later and dominated the north, founding the future metropolis of Chicago in the 1830s. The coming of the railroads in the 1850s made highly profitable the rich prairie farmlands in central Illinois, attracting large numbers of immigrant farmers from Germany and Sweden. Northern Illinois provided major support for Illinoisans Abraham Lincoln and Ulysses S. Grant during the American Civil War. By 1900, factories were being rapidly built in the northern cities, along with coal mines in central and southern areas. This industrialization attracted large numbers of immigrants from Eastern and Southern Europe, and also led to the state's material contribution as a major arsenal in both world wars. In addition to immigrants from other countries, large numbers of blacks left the cotton fields of the South to come to Chicago, where they developed a famous jazz culture.

The state is named for the Illinois River which was named by French explorers after the indigenous Illiniwek people, a consortium of Algonquian tribes that thrived in the area. The word Illiniwek means "tribe of superior men."

The northeastern border of Illinois is Lake Michigan. Its eastern border with Indiana is all of the land west of the Wabash River, and a north-south line above Post Vincennes, or 87° 30' west longitude. Its northern border with Wisconsin is fixed at 42° 30' latitude. Its western border with Missouri and Iowa is the Mississippi River. Its southern border with Kentucky is the Ohio River. Illinois also borders Michigan, but only via a water boundary in Lake Michigan.

Though Illinois lies entirely in the Interior Plains, it has three major geographical divisions. The first is Chicagoland, including the city of Chicago, its suburbs, and the adjoining exurban area into which the metropolis is expanding. Two out of three Illinoisans live in this region. This region includes a few counties in Indiana and Wisconsin and stretches across much of northern Illinois toward the Iowa border, generally along Interstates 80 and 90. This region is cosmopolitan, densely populated, industrialized, and settled by a variety of ethnic groups. Cook County is the most populous county in the state, with over 5.3 million residents in 2004.

Southward and westward, the second major division is central Illinois, an area of mostly flat prairie. The western section (west of the Illinois River) was originally part of the Military Tract of 1812 and forms the distinctive western bulge of state. Known as the Heart of Illinois, it is characterized by small towns and mid-sized cities. Agriculture, particularly corn and soybeans, as well as educational institutions and manufacturing centers, figure prominently. Major cities include Peoria–the third largest metropolitan area in Illinois at 370,000, Springfield–the state capital, Decatur, Bloomington-Normal and Champaign-Urbana.

The third division is southern Illinois, comprising the area south of U.S. Route 50, and including Little Egypt, near the juncture of the Mississippi River and Ohio River. This region can be distinguished from the other two by its warmer climate, different mix of crops (including some cotton farming in the past), more rugged topography (the southern tip is unglaciated with the remainder glaciated during the Illinoian Age and earlier ages), as well as small-scale oil deposits and coal mining. The area is a little more populated than the central part of the state with the population centered in two areas. First, the Illinois suburbs of St. Louis comprise the second most populous metropolitan area in Illinois with nearly 600,000 inhabitants, and are known collectively as the Metro-East. Second, the Carbondale, Marion, West Frankfort, Herrin, Murphysboro area, is home to around 200,000 residents.

The region outside of the Chicago Metropolitan area is often described as "downstate Illinois". However, residents of central and southern Illinois view their regions as geographically and culturally distinct, and do not necessarily use this term.

In extreme northwestern Illinois, the Driftless Zone, a region of unglaciated and therefore higher and more rugged topography, occupies a small part of the state. Charles Mound, located in this region, is the state's highest natural elevation above sea level at 1,235 feet (376 m). The highest true elevation in Illinois is the Sears Tower with an elevation at the top of its roof of approximately 2,030 feet (the elevation of Chicago is approximately 580 feet and the height of the roof is approximately 1450 feet).