American Indians of successive cultures lived along the waterways of the Illinois area for thousands of years before the arrival of Europeans. The Koster Site has been excavated and demonstrates 7,000 years of continuous habitation. Cahokia, the largest regional chiefdom and Urban Center of the Pre-Columbian Mississippian culture, was located near present-day Collinsville, Illinois. They built an urban complex of more than 100 platform and burial mounds, a 50-acre (20 ha) plaza larger than 35 football fields, and a woodhenge of sacred cedar, all in a planned design expressing the culture's cosmology. Monks Mound, the center of the site, is the largest Pre-Columbian structure north of the Valley of Mexico. It is 100 ft (30 m) high, 951 ft (290 m) long, 836 ft (255 m) wide, and covers 13.8 acres (5.6 ha). It contains about 814,000 cu yd (622,000 m) of earth. It was topped by a structure thought to have measured about 105 ft (32 m) in length and 48 ft (15 m) in width, covered an area 5,000 sq ft (460 m), and been as much as 50 ft (15 m) high, making its peak 150 ft (46 m) above the level of the plaza. The finely crafted ornaments and tools recovered by archaeologists at Cahokia include elaborate ceramics, finely sculptured stonework, carefully embossed and engraved copper and mica sheets, and one funeral blanket for an important chief fashioned from 20,000 shell beads. These artifacts indicate that Cahokia was truly an urban center, with clustered housing, markets, and specialists in toolmaking, hide dressing, potting, jewelry making, shell engraving, weaving and salt making.

The civilization vanished in the 15th century for unknown reasons, but historians and archeologists have speculated that the people depleted the area of resources. Many indigenous tribes engaged in constant warfare. According to Suzanne Austin Alchon, "At one site in the central Illinois River valley, one third of all adults died as a result of violent injuries." The next major power in the region was the Illinois Confederation or Illini, a political alliance. As the Illini declined during the Beaver Wars era, members of the Algonquian-speaking Potawatomi, Miami, Sauk, and other tribes including the Fox (Mesquakie), Ioway, Kickapoo, Mascouten, Piankashaw, Shawnee, Wea, and Winnebago (Ho-Chunk) came into the area from the east and north around the Great Lakes.

French explorers Jacques Marquette and Louis Jolliet explored the Illinois River in 1673. Marquette soon after founded a mission at the Grand Village of the Illinois in Illinois Country. In 1680, French explorers under René-Robert Cavelier, Sieur de La Salle and Henri de Tonti constructed a fort at the site of present-day Peoria, and in 1682, a fort atop Starved Rock in today's Starved Rock State Park. French Empire Canadiens came south to settle particularly along the Mississippi River, and Illinois was part of first New France, and then of La Louisiane until 1763, when it passed to the British with their defeat of France in the Seven Years' War. The small French settlements continued, although many French migrated west to Ste. Genevieve and St. Louis, Missouri, to evade British rule.

A few British soldiers were posted in Illinois, but few British or American settlers moved there, as the Crown made it part of the territory reserved for Indians west of the Appalachians, and then part of the British Province of Quebec. In 1778, George Rogers Clark claimed Illinois County for Virginia. In a compromise, Virginia (and other states that made various claims) ceded the area to the new United States in the 1780s and it became part of the Northwest Territory, administered by the federal government and later organized as states.

The Illinois-Wabash Company was an early claimant to much of Illinois. The Illinois Territory was created on February 3, 1809, with its capital at Kaskaskia, an early French settlement.

During the discussions leading up to Illinois's admission to the Union, the proposed northern boundary of the state was moved twice. The original provisions of the Northwest Ordinance had specified a boundary that would have been tangent to the southern tip of Lake Michigan. Such a boundary would have left Illinois with no shoreline on Lake Michigan at all. However, as Indiana had successfully been granted a 10 mi (16 km) northern extension of its boundary to provide it with a usable lakefront, the original bill for Illinois statehood, submitted to Congress on January 23, 1818, stipulated a northern border at the same latitude as Indiana's, which is defined as 10 miles north of the southernmost extremity of Lake Michigan. However, the Illinois delegate, Nathaniel Pope, wanted more, and lobbied to have the boundary moved further north. The final bill passed by Congress included an amendment to shift the border to 42° 30' north, which is approximately 51 mi (82 km) north of the Indiana northern border. This shift added 8,500 sq mi (22,000 km²) to the state, including the lead mining region near Galena. More importantly, it added nearly 50 miles of Lake Michigan shoreline and the Chicago River. Pope and others envisioned a canal that would connect the Chicago and Illinois rivers and thus connect the Great Lakes to the Mississippi.

In 1818, Illinois became the 21st U.S. state. The capital remained at Kaskaskia, headquartered in a small building rented by the state. In 1819, Vandalia became the capital, and over the next 18 years, three separate buildings were built to serve successively as the capitol building. In 1837, the state legislators representing Sangamon County, under the leadership of state representative Abraham Lincoln, succeeded in having the capital moved to Springfield, where a fifth capitol building was constructed. A sixth capitol building was erected in 1867, which continues to serve as the Illinois capitol today.

Though it was ostensibly a "free state", there was nonetheless slavery in Illinois. The ethnic French had owned black slaves since the 1720s, and American settlers had already brought slaves into the area from Kentucky. Slavery was nominally banned by the Northwest Ordinance, but that was not enforced for those already holding slaves. When Illinois became a sovereign state in 1818, the Ordinance no longer applied, and about 900 slaves were held in the state. As the southern part of the state, later known as "Egypt" or "Little Egypt", was largely settled by migrants from the South, the section was hostile to free blacks. Settlers were allowed to bring slaves with them for labor, but, in 1822, state residents voted against making slavery legal. Still, most residents opposed allowing free blacks as permanent residents. Some settlers brought in slaves seasonally or as house servants. The Illinois Constitution of 1848 was written with a provision for exclusionary laws to be passed. In 1853, John A. Logan helped pass a law to prohibit all African Americans, including freedmen, from settling in the state.

The winter of 1830–1831 is called the "Winter of the Deep Snow"; a sudden, deep snowfall blanketed the state, making travel impossible for the rest of the winter, and many travelers perished. Several severe winters followed, including the "Winter of the Sudden Freeze". On December 20, 1836, a fast-moving cold front passed through, freezing puddles in minutes and killing many travelers who could not reach shelter. The adverse weather resulted in crop failures in the northern part of the state. The southern part of the state shipped food north, and this may have contributed to its name: "Little Egypt", after the Biblical story of Joseph in Egypt supplying grain to his brothers.

In 1832, the Black Hawk War was fought in Illinois and present-day Wisconsin between the United States and the Sauk, Fox (Meskwaki), and Kickapoo Indian tribes. It represents the end of Indian resistance to white settlement in the Chicago region. The Indians had been forced to leave their homes and move to Iowa in 1831; when they attempted to return, they were attacked and eventually defeated by U.S. militia. The survivors were forced back to Iowa.

By 1839, the Latter Day Saints had founded a utopian city called Nauvoo. Located in Hancock County along the Mississippi River, Nauvoo flourished, and soon rivaled Chicago for the position of the state's largest city. But in 1844, the Latter Day Saint movement founder Joseph Smith was killed in the Carthage Jail, about 30 miles away from Nauvoo. Following a succession crisis (Latter Day Saints), Brigham Young led most Latter Day Saints out of Illinois in a mass exodus to present-day Utah; after close to six years of rapid development, Nauvoo rapidly declined afterward.

After it was established in 1833, Chicago gained prominence as a Great Lakes port, and then as an Illinois and Michigan Canal port after 1848, and as a rail hub soon afterward. By 1857, Chicago was Illinois's largest city. With the tremendous growth of mines and factories in the state in the 19th century, Illinois was the ground for the formation of labor unions in the United States.

In 1847, after lobbying by Dorothea L. Dix, Illinois became one of the first states to establish a system of state-supported treatment of mental illness and disabilities, replacing local almshouses. Dix came into this effort after having met J. O. King, a Jacksonville, Illinois businessman, who invited her to Illinois, where he had been working to build an asylum for the insane. With the lobbying expertise of Dix, plans for the Jacksonville State Hospital (now known as the Jacksonville Developmental Center) were signed into law on March 1, 1847.

During the American Civil War, Illinois ranked fourth in men who served (more than 250,000) in the Union Army, a figure surpassed by only New York, Pennsylvania, and Ohio. Beginning with President Abraham Lincoln's first call for troops and continuing throughout the war, Illinois mustered 150 infantry regiments, which were numbered from the 7th to the 156th regiments. Seventeen cavalry regiments were also gathered, as well as two light artillery regiments. The town of Cairo, at the southern tip of the state at the confluence of the Mississippi and Ohio Rivers, served as a strategically important supply base and training center for the Union army. For several months, both General Grant and Admiral Foote had headquarters in Cairo.

During the Civil War, and more so afterwards, Chicago's population skyrocketed, which increased its prominence. The Pullman Strike and Haymarket Riot, in particular, greatly influenced the development of the American labor movement. From Sunday, October 8, 1871, until Tuesday, October 10, 1871, the Great Chicago Fire burned in downtown Chicago, destroying four sq mi (10 km).

At the turn of the 20th century, Illinois had a population of nearly 5 million. Many people from other parts of the country were attracted to the state by employment caused by the expanding industrial base. Whites were 98% of the state's population. Bolstered by continued immigration from southern and eastern Europe, and by the African-American Great Migration from the South, Illinois grew and emerged as one of the most important states in the union. By the end of the century, the population had reached 12.4 million.

The Century of Progress World's fair was held at Chicago in 1933. Oil strikes in Marion County and Crawford County led to a boom in 1937, and by 1939, Illinois ranked fourth in U.S. oil production. Illinois manufactured 6.1 percent of total United States military armaments produced during World War II, ranking seventh among the 48 states. Chicago became an ocean port with the opening of the Saint Lawrence Seaway in 1959. The seaway and the Illinois Waterway connected Chicago to both the Mississippi River and the Atlantic Ocean. In 1960, Ray Kroc opened the first McDonald's franchise in Des Plaines (which still exists as a museum, with a working McDonald's across the street).

Illinois had a prominent role in the emergence of the nuclear age. In 1942, as part of the Manhattan Project, the University of Chicago conducted the first sustained nuclear chain reaction. In 1957, Argonne National Laboratory, near Chicago, activated the first experimental nuclear power generating system in the United States. By 1960, the first privately financed nuclear plant in the United States, Dresden 1, was dedicated near Morris. In 1967, Fermilab, a national nuclear research facility near Batavia, opened a particle accelerator, which was the world's largest for over 40 years. With eleven plants currently operating, Illinois leads all states in the amount of electricity generated from nuclear power.

In 1961, Illinois became the first state in the nation to adopt the recommendation of the American Law Institute and pass a comprehensive criminal code revision that repealed the law against sodomy. The code also abrogated common law crimes and established an age of consent of 18. The state's fourth constitution was adopted in 1970, replacing the 1870 document.

The first Farm Aid concert was held in Champaign to benefit American farmers, in 1985. The worst upper Mississippi River flood of the century, the Great Flood of 1993, inundated many towns and thousands of acres of farmland.

On August 28, 2017, Illinois Governor Bruce Rauner signed a bill into law that prohibited state and local police from arresting anyone solely due to their immigration status or due to federal detainers. Some fellow Republicans criticized Rauner for his action, claiming the bill made Illinois a sanctuary state.

Illinois is located in the Midwest region of the United States and is one of the eight states in the Great Lakes region of North America (which also includes Ontario, Canada).

Illinois's eastern border with Indiana consists of a north–south line at 87° 31′ 30″ west longitude in Lake Michigan at the north, to the Wabash River in the south above Post Vincennes. The Wabash River continues as the eastern/southeastern border with Indiana until the Wabash enters the Ohio River. This marks the beginning of Illinois's southern border with Kentucky, which runs along the northern shoreline of the Ohio River. Most of the western border with Missouri and Iowa is the Mississippi River; Kaskaskia is an exclave of Illinois, lying west of the Mississippi and reachable only from Missouri. The state's northern border with Wisconsin is fixed at 42° 30′ north latitude. The northeastern border of Illinois lies in Lake Michigan, within which Illinois shares a water boundary with the state of Michigan, as well as Wisconsin and Indiana.

Though Illinois lies entirely in the Interior Plains, it does have some minor variation in its elevation. In extreme northwestern Illinois, the Driftless Area, a region of unglaciated and therefore higher and more rugged topography, occupies a small part of the state. Southern Illinois includes the hilly areas around the Shawnee National Forest.

Charles Mound, located in the Driftless region, has the state's highest natural elevation above sea level at 1,235 ft (376 m). Other highlands include the Shawnee Hills in the south, and there is varying topography along its rivers; the Illinois River bisects the state northeast to southwest. The floodplain on the Mississippi River from Alton to the Kaskaskia River is known as the American Bottom.

Illinois has three major geographical divisions. Northern Illinois is dominated by Chicago metropolitan area, or Chicagoland, which is the city of Chicago and its suburbs, and the adjoining exurban area into which the metropolis is expanding. As defined by the federal government, the Chicago metro area includes several counties in Illinois, Indiana, and Wisconsin, and has a population of over 9.8 million. Chicago itself is a cosmopolitan city, densely populated, industrialized, the transportation hub of the nation, and settled by a wide variety of ethnic groups. The city of Rockford, Illinois's third-largest city and center of the state's fourth largest metropolitan area, sits along Interstates 39 and 90 some 75 mi (121 km) northwest of Chicago. The Quad Cities region, located along the Mississippi River in northern Illinois, had a population of 381,342 in 2011.

The midsection of Illinois is the second major division, called Central Illinois. Historically prairie, it is now mainly agricultural and known as the Heart of Illinois. It is characterized by small towns and medium–small cities. The western section (west of the Illinois River) was originally part of the Military Tract of 1812 and forms the conspicuous western bulge of the state. Agriculture, particularly corn and soybeans, as well as educational institutions and manufacturing centers, figure prominently in Central Illinois. Cities include Peoria; Springfield, the state capital; Quincy; Decatur; Bloomington-Normal; and Champaign-Urbana.

The third division is Southern Illinois, comprising the area south of U.S. Route 50, including Little Egypt, near the juncture of the Mississippi River and Ohio River. Southern Illinois is the site of the ancient city of Cahokia, as well as the site of the first state capital at Kaskaskia, which today is separated from the rest of the state by the Mississippi River. This region has a somewhat warmer winter climate, different variety of crops (including some cotton farming in the past), more rugged topography (due to the area remaining unglaciated during the Illinoian Stage, unlike most of the rest of the state), as well as small-scale oil deposits and coal mining. The Illinois suburbs of St. Louis, such as East St. Louis, are located in this region, and collectively, they are known as the Metro-East. The other somewhat significant concentration of population in Southern Illinois is the Carbondale-Marion-Herrin, Illinois Combined Statistical Area centered on Carbondale and Marion, a two-county area that is home to 123,272 residents. A portion of southeastern Illinois is part of the extended Evansville, Indiana, Metro Area, locally referred to as the Tri-State with Indiana and Kentucky. Seven Illinois counties are in the area.

In addition to these three, largely latitudinally defined divisions, all of the region outside the Chicagoland area is often called "downstate" Illinois. This term is flexible, but is generally meant to mean everything outside the influence of the Chicago area. Thus, some cities in Northern Illinois, such as DeKalb, which is west of Chicago, and Rockford—which is actually north of Chicago—are sometimes incorrectly considered to be 'downstate'.

Illinois has a climate that varies widely throughout the year. Because of its nearly 400-mile distance between its northernmost and southernmost extremes, as well as its mid-continental situation, most of Illinois has a humid continental climate (Köppen climate classification Dfa), with hot, humid summers and cold winters. The southern part of the state, from about Carbondale southward, has a humid subtropical climate (Koppen Cfa), with more moderate winters. Average yearly precipitation for Illinois varies from just over 48 in (1,219 mm) at the southern tip to around 35 in (889 mm) in the northern portion of the state. Normal annual snowfall exceeds 38 in (965 mm) in the Chicago area, while the southern portion of the state normally receives less than 14 in (356 mm). The all-time high temperature was 117 °F (47 °C), recorded on July 14, 1954, at East St. Louis, and the all-time low temperature was −38 °F (−39 °C), recorded on January 31, 2019, during the January 2019 North American cold wave at a weather station near Mount Carroll, and confirmed on March 5, 2019. This followed the previous record of −36 °F (−38 °C) recorded on January 5, 1999, near Congerville. Prior to the Mount Carroll record, a temperature of −37 °F (−38 °C) was recorded on January 15, 2009, at Rochelle, but at a weather station not subjected to the same quality control as official records.

Illinois averages approximately 51 days of thunderstorm activity a year, which ranks somewhat above average in the number of thunderstorm days for the United States. Illinois is vulnerable to tornadoes, with an average of 35 occurring annually, which puts much of the state at around five tornadoes per 10,000 sq mi (30,000 km) annually. While tornadoes are no more powerful in Illinois than other states, some of Tornado Alley's deadliest tornadoes on record have occurred in the state. The Tri-State Tornado of 1925 killed 695 people in three states; 613 of the victims died in Illinois.

The United States Census Bureau found that the population of Illinois was 12,812,508 in the 2020 United States census, moving from the fifth-largest state to the sixth-largest state (losing out to Pennsylvania). Illinois' population slightly declined in 2020 from the 2010 United States census by just over 18,000 residents and the overall population was quite higher than recent census estimates.

Illinois is the most populous state in the Midwest region. Chicago, the third-most populous city in the United States, is the center of the Chicago metropolitan area or Chicagoland, as this area is nicknamed. Although Chicagoland comprises only 9% of the land area of the state, it contains 65% of the state's residents. The losses of population anticipated from the 2020 census results do not arise from the Chicago metro area; rather the declines are from the Downstate counties.

According to 2019 U.S. Census Bureau estimates, Illinois's population was 71.4% White (60.7% Non-Hispanic White), 5.6% Asian, 0.2% Some Other Race, 13.9% Black or African American, 0.1% Native Americans and Alaskan Native, 0.1% Pacific Islander and 2.0% from two or more races. The White population continues to remain the largest racial category in Illinois as Hispanics primarily identify as White (61.1%) with others identifying as Some Other Race (32.0%), Multiracial (4.3%), Black (1.4%), American Indian and Alaskan Native (0.2%), Asian (0.1%), and Hawaiian and Pacific Islander (0.1%). By ethnicity, 17.5% of the total population is Hispanic-Latino (of any race) and 82.5% is Non-Hispanic (of any race). If treated as a separate category, Hispanics are the largest minority group in Illinois.

The state's most populous ethnic group, non-Hispanic white, has declined from 83.5% in 1970 to 60.90% in 2018. As of 2011, 49.4% of Illinois's population younger than age 1 were minorities (Note: Children born to white Hispanics or to a sole full or partial minority parent are counted as minorities).

At the 2007 estimates from the U.S. Census Bureau, there were 1,768,518 foreign-born inhabitants of the state or 13.8% of the population, with 48.4% from Latin America, 24.6% from Asia, 22.8% from Europe, 2.9% from Africa, 1.2% from Canada, and 0.2% from Oceania. Of the foreign-born population, 43.7% were naturalized U.S. citizens, and 56.3% were not U.S. citizens. In 2007, 6.9% of Illinois's population was reported as being under age 5, 24.9% under age 18 and 12.1% were age 65 and over. Females made up approximately 50.7% of the population.

According to the 2007 estimates, 21.1% of the population had German ancestry, 13.3% had Irish ancestry, 8% had British ancestry, 7.9% had Polish ancestry, 6.4% had Italian ancestry, 4.6% listed themselves as American, 2.4% had Swedish ancestry, 2.2% had French ancestry, other than Basque, 1.6% had Dutch ancestry, and 1.4% had Norwegian ancestry. Illinois also has large numbers of African Americans and Latinos (mostly Mexicans and Puerto Ricans).

Chicago, along the shores of Lake Michigan, is the nation's third largest city. In 2000, 23.3% of Illinois's population lived in the city of Chicago, 43.3% in Cook County, and 65.6% in the counties of the Chicago metropolitan area: Will, DuPage, Kane, Lake, and McHenry counties, as well as Cook County. The remaining population lives in the smaller cities and rural areas that dot the state's plains. As of 2000, the state's center of population was at 41°16′42″N 88°22′49″W / 41.278216°N 88.380238°W / 41.278216; -88.380238, located in Grundy County, northeast of the village of Mazon.

Births do not add up, because Hispanics are counted both by ethnicity and by race.

Chicago is the largest city in the state and the third-most populous city in the United States, with its 2020 population of 2,746,388. The U.S. Census Bureau currently lists seven other cities with populations of over 100,000 within Illinois. Based upon the U.S. Census Bureau's official 2010 population: Aurora, a Chicago satellite town that eclipsed Rockford for the title of second-most populous city in Illinois; its 2010 population was 197,899. Rockford, at 152,871, is the third-largest city in the state, and is the largest city in the state not located within the Chicago suburbs. Joliet, located in metropolitan Chicago, is the fourth-largest city in the state, with a population of 147,433. Naperville, a suburb of Chicago, is fifth with 141,853. Naperville and Aurora share a boundary along Illinois Route 59. Springfield, the state's capital, comes in as sixth-most populous with 117,352 residents. Peoria, which decades ago was the second-most populous city in the state, is seventh with 115,007. The eighth-largest and final city in the 100,000 club is Elgin, a northwest suburb of Chicago, with a 2010 population of 108,188.

The most populated city in the state south of Springfield is Belleville, with 44,478 people at the 2010 census. It is located in the Illinois portion of Greater St. Louis (often called the Metro-East area), which has a rapidly growing population of over 700,000.

Other major urban areas include the Champaign-Urbana Metropolitan Area, which has a combined population of almost 230,000 people, the Illinois portion of the Quad Cities area with about 215,000 people, and the Bloomington-Normal area with a combined population of over 165,000.

The official language of Illinois is English, although between 1923 and 1969, state law gave official status to "the American language". Nearly 80% of people in Illinois speak English natively, and most of the rest speak it fluently as a second language. A number of dialects of American English are spoken, ranging from Inland Northern American English and African-American English around Chicago, to Midland American English in Central Illinois, to Southern American English in the far south.

Over 20% of Illinoians speak a language other than English at home, of which Spanish is by far the most widespread, at more than 12% of the total population. A sizeable number of Polish speakers is present in the Chicago Metropolitan Area. Illinois Country French has mostly gone extinct in Illinois, although it is still celebrated in the French Colonial Historic District.

Roman Catholics constitute the single largest religious denomination in Illinois; they are heavily concentrated in and around Chicago, and account for nearly 30% of the state's population. However, taken together as a group, the various Protestant denominations comprise a greater percentage of the state's population than do Catholics. In 2010, Catholics in Illinois numbered 3,648,907. The largest Protestant denominations were the United Methodist Church with 314,461 and the Southern Baptist Convention with 283,519 members. Illinois has one of the largest concentrations of Missouri Synod Lutherans in the United States.

Illinois played an important role in the early Latter Day Saint movement, with Nauvoo, Illinois becoming a gathering place for Mormons in the early 1840s. Nauvoo was the location of the succession crisis, which led to the separation of the Mormon movement into several Latter Day Saint sects. The Church of Jesus Christ of Latter-day Saints, the largest of the sects to emerge from the Mormon schism, has more than 55,000 adherents in Illinois today.

A significant number of adherents of other Abrahamic faiths can be found in Illinois. Largely concentrated in the Chicago metropolitan area, followers of the Muslim, Baháʼí, and Jewish religions all call the state home. Muslims constituted the largest non-Christian group, with 359,264 adherents. Illinois has the largest concentration of Muslims by state in the country, with 2,800 Muslims per 100,000 citizens.

The largest and oldest surviving Baháʼí House of Worship in the world is located on the shores of Lake Michigan in Wilmette, Illinois, one of eight continental Baháʼí House of Worship. It serves as a space for people of all backgrounds and religions to gather, meditate, reflect, and pray, expressing the Baháʼí principle of the oneness of religions. The Chicago area has a very large Jewish community, particularly in the suburbs of Skokie, Buffalo Grove, Highland Park, and surrounding suburbs. Former Chicago Mayor Rahm Emanuel was the Windy City's first Jewish mayor.

Chicago is also home to a very large population of Hindus, Sikhs, Jains, and Buddhists.