Stevens recorded and produced the album at multiple venues in New York City using low-fidelity studio equipment and a variety of instruments between late 2004 and early 2005.

The artwork and lyrics explore the history, culture, art, and geography of the state—Stevens developed them after analyzing criminal, literary, and historical documents.

The varied instrumentation and experimental songwriting on the album invoked comparisons to work by Steve Reich, Neil Young, and the Cure.

Stevens launched his 50-state project in 2003 with the album Michigan and chose to focus on Illinois with this recording because "it wasn't a great leap", and he liked the state because he considered it the "center of gravity" for the American Midwest.

[1] Before creating the album, Stevens read literature by Illinois authors Saul Bellow and Carl Sandburg,[1] and studied immigration records[2] and history books for the state—he made the deliberate decision to avoid current events and focused on historical themes.

[5] Although he began work in 2004[6] on Oregon-themed songs and briefly considered releasing a Rhode Island-themed 7-inch,[4] Stevens has since not released another album focused on a state, saying in a November 2009 interview with Paste that "the whole premise was such a joke,"[7] and telling Andrew Purcell of The Guardian in October 2009 "I have no qualms about admitting [the fifty states project] was a promotional gimmick.

[13] After consulting with Michael Kaufmann and Lowell Brams of Asthmatic Kitty about the amount of material he had recorded, Stevens decided against a double album, saying that would be "arrogant".

Among these outtakes are three separate recordings of the song "Chicago"—including the "Multiple Personality Disorder Version", which was produced during a subsequent tour.

[5] He was supported on some dates by opening acts Liz Janes (who is also signed to Asthmatic Kitty) and Laura Veirs[17] as well as Illinois collaborator Shara Nova's solo project My Brightest Diamond.

[19] During the 2006 dates, Stevens and his band transitioned from wearing University of Illinois-themed outfits to butterfly suits and bird wings.

[20] Reviewers have noted similarities between this album and those of musicians and composers in several musical genres—from pop to contemporary classical, even show tunes and jazz-based time signatures.

[29] Reviewers of Illinois have compared Stevens' style to Steve Reich,[30] Vince Guaraldi, the Danielson Famile, Neil Young,[29] Nick Drake, and Death Cab for Cutie.

Club referred to some of the vocal work as "regressively twee communalism", but found Stevens' music overall to be "highly developed".

Several lyrics make explicit references to events in his life: "[w]hen the swingset hit his head" refers to an event in Gacy's childhood, when a swing hit his head and caused a blood clot in his brain;[11][28] "He dressed up like a clown for them / with his face paint white and red" alludes to the nickname given to Gacy—the "Killer Clown";[2] and "He put a cloth on their lips / Quiet hands, quiet kiss on the mouth" references Gacy's use of chloroform to subdue and molest his victims.

[37] Some references to Decatur included in the song were alligator sightings in the area, the equipment manufacturer Caterpillar, and a flood that exhumed a graveyard of soldiers from the Civil War.

As a Christian, he has written and recorded music about spiritual themes throughout his career—particularly on the 2004 album Seven Swans—and prefers to talk about religious topics through song rather than directly in interviews or public statements.

[38] "Casimir Pulaski Day" describes the death of a romantic partner due to bone cancer (despite Bible study prayers for healing), and the narrator questions God in the process.

[20] Divya Srinivasan created the album artwork,[11] depicting a variety of Illinois-related themes, including Abraham and Mary Todd Lincoln, the Sears Tower, and Black Hawk.

[40] Shortly after the release of the album, reports arose that DC Comics had issued a cease and desist letter to Asthmatic Kitty because of the depiction of Superman on the cover.

[41][42] However, on October 4, 2005, Asthmatic Kitty announced that there had been no cease and desist letter; the record company's own lawyers had warned about the copyright infringement.

[25] Tim Jonze of NME called Illinois "a brainy little fucker" and described Stevens as "prolific, intelligent and—most importantly—brimming with heart-wrenching melodies.

"[2] Rob Sheffield of Rolling Stone responded favorably to the album, praising the "over-the-top arrangements" and Stevens' "breathy, gentle voice" as well as the personal nature of songs such as "Chicago" and "Casimir Pulaski Day", but criticized "John Wayne Gacy, Jr.", stating that it "symbolizes nothing about American life except the existence of creative-writing workshops".

"[34] Amanda Petrusich of Pitchfork described Illinois as "strange and lush, as excessive and challenging as its giant, gushing song titles.

"[28] Dave Simpson of The Guardian echoed this sentiment by saying that the music sounds like "The Polyphonic Spree produced by Brian Eno.

[49] Jesse Jarnow of Paste praised the playful nature of Illinois, commenting that it had "sing-song" melodies and "jaunty" orchestrations.

[58] Critic Andy Gill dubbed Illinois "an extraordinary achievement" in Uncut,[59] and in a separate review for The Independent added that it "makes most other albums seem small-minded and, ironically, rather parochial.

"[51] Catherine Lewis of The Washington Post responded favorably to the album, stating that it has well-written lyrics, comparing Stevens' rhyming to that of Stephin Merritt.

[85][86] In August 2017, the album was certified gold by the Recording Industry Association of America (RIAA), signifying shipments in excess of 500,000 copies in the United States.

The show features new arrangements for a live band and three voices, and aims to "lead [the audience] on a mighty journey through the American heartland, from campfire storytelling to the edges of the cosmos".