Fall Out Boy is an American rock band formed in Wilmette, Illinois, a suburb of Chicago, in 2001. The band consists of lead vocalist and rhythm guitarist Patrick Stump, bassist Pete Wentz, lead guitarist Joe Trohman, and drummer Andy Hurley. The band originated from Chicago's hardcore punk scene and was formed by Wentz and Trohman as a pop-punk side project; Stump joined shortly thereafter. The group went through a succession of drummers before Hurley joined. Their debut album, Take This to Your Grave (2003), became an underground success and helped the band gain a dedicated fan base through heavy touring.

With Wentz as the band's lyricist and Stump as the primary composer, Fall Out Boy's 2005 major-label breakthrough, From Under the Cork Tree, produced two hit singles, "Sugar, We're Goin Down" and "Dance, Dance". It went double platinum, transforming the group into superstars and making Wentz a celebrity and tabloid fixture. Fall Out Boy received a Best New Artist nomination at the 2006 Grammy Awards. Infinity on High (2007) debuted at number one on the Billboard 200 with 260,000 first week sales. It produced two worldwide hit singles, "This Ain't a Scene, It's an Arms Race" and "Thnks fr th Mmrs". Their following album, Folie à Deux (2008), was a commercial disappointment and received a mixed response. Following the release of Believers Never Die – Greatest Hits in 2009, the band went on hiatus and the members worked on side projects.

The band reunited with Save Rock and Roll (2013), which became Fall Out Boy's second number-one album, and included the top 20 single "My Songs Know What You Did in the Dark (Light Em Up)". The same year, the band released the EP PAX AM Days, consisting of eight punk-influenced tracks that were recorded during a two-day session with producer Ryan Adams. The band's sixth studio album, American Beauty/American Psycho (2015) peaked at number one on the Billboard 200, and spawned the top-10 hit "Centuries" and the single "Uma Thurman" which reached No. 22 on the Billboard Hot 100. This was followed by their first remix album Make America Psycho Again, which featured remixes of all original tracks from American Beauty/American Psycho by a different artist on each song, including Migos and Wiz Khalifa.

The band's seventh studio album Mania (2018), also peaked at No. 1, making it the band's fourth No. 1 album and sixth consecutive Top 10 album. The Mania tour included a show at Wrigley Field, marking their first headlining stadium show. In 2018, Fall Out Boy also received their second Grammy nomination for Best Rock Album for Mania. On January 18, 2023, the group announced its eighth studio album, So Much (for) Stardust, which was released on March 24.

Fall Out Boy was formed in 2001 in the Chicago suburb of Wilmette, Illinois by friends Pete Wentz and Joe Trohman. Wentz was a "visible fixture" of the relatively small Chicago hardcore scene of the late 1990s, performing in groups such as Birthright, Extinction and First Born. He was also part of the metalcore band Arma Angelus and the more political Racetraitor, "a band that managed to land the covers of Maximumrocknroll and Heartattack fanzines before releasing a single note of music". Wentz was growing dissatisfied with the changing mores of the community, which he viewed as a transition from political activism to an emphasis on moshing and breakdowns. With enthusiasm in Arma Angelus waning, he created a pop-punk side project with Trohman that was intended to be "easy and escapist". Patrick Stump was the drummer for grindcore band Xgrinding processX and a host of other bands that "never really managed." At a Borders bookstore in Wilmette, while Trohman was discussing Neurosis with a friend, Stump interrupted them to correct their classification of the band; the ensuing conversation soon shifted to Trohman and Wentz's new project. Stump, viewing it as an opportunity to try out with "local hardcore celebrity" Wentz, directed Trohman to his MP3.com page, which contained sung-through acoustic recordings. Stump intended to try out as a drummer, but Trohman urged him to bring out his acoustic guitar; Stump impressed Trohman and Wentz with songs from Saves the Day's Through Being Cool. While Wentz wanted Racetraitor bandmate Andy Hurley to join the group as drummer, Hurley appeared uninterested and too busy at the time.

The band's first public performance came in a cafeteria at DePaul University alongside Stilwell and another group that performed Black Sabbath's self-titled debut album in its entirety. The band's only performance with guitarist John Flamandan and original drummer Ben Rose was in retrospect described as "goofy" and "bad", but Trohman made an active effort to make the band work, picking up members for practice. During this performance, they introduced themselves as Forget Me Not. Wentz and Stump argued over band names; the former favored verbose, tongue-in-cheek names, while the latter wanted to reference Tom Waits in name. After creating a short list of names that included "Fall Out Boy", a fictional character from The Simpsons and Bongo Comics, friends voted on the name. The band's second performance, at a southern Illinois university with the Killing Tree, began with Wentz introducing the band under a name Stump recalled as "very long". According to Stump, an audience member yelled out, "Fuck that, no, you're Fall Out Boy!", and the band were credited later in the show under that name by Killing Tree frontman Tim McIlrath. As the group looked up to McIlrath, and Trohman and Stump were "die-hard" Simpsons fans, the name stuck. The group's first cassette tape demo was recorded in Ben Rose's basement, but the band later set off for Wisconsin to record a proper demo with 7 Angels 7 Plagues drummer Jared Logan, whom Wentz knew through connections in the hardcore scene.

Several more members passed through the group, including drummer Mike Pareskuwicz of Subsist and guitarist T.J. "Racine" Kunasch. While Stump at this point felt uninterested in the group, Wentz, according to Uprising Records owner Sean Muttaqi, viewed the group as "the thing that would make him famous. He had a clear vision." Wentz was "singularly focused on taking things to the next level" and began promoting the band via early social media. Muttaqi got word of the demo and wanted to release half of it as a split extended play with Hurley's band Project Rocket, which the band viewed as competition. Uprising wanted to release an album with the emerging band, which had only written three songs at that point. With the help of Logan, the group attempted to put together a collection of songs in two days and recorded them as Fall Out Boy's Evening Out with Your Girlfriend. The rushed recording experience and underdeveloped songs left the band dissatisfied. When the band set off to Smart Studios in Madison, Wisconsin to record three songs for a possible split 7-inch with 504 Plan, engineer Sean O'Keefe suggested the band record the songs with Hurley. Hurley was recording an EP with his new group the Kill Pill in Chicago on the same day, but raced to Madison to play drums for Fall Out Boy. "It was still a fill-in thing but when Andy sat in, it just felt different. It was one of those 'a-ha' moments", recalled Wentz.

The band booked a two-week tour with Spitalfield, but Pareskuwicz was unable to get time off from work and Kunasch was kicked out of the band as the group "had all gotten sick of him". Kunasch was temporarily replaced by friend Brandon Hamm on guitar, alongside drummer Chris Envy from the recently disbanded Showoff, but both quit prior to the kickoff of the tour. The band invited Hurley to fill-in once more, while Stump borrowed one of Trohman's guitars for the trek. While most shows were cancelled, the band played any show possible: "Let's just get on whatever show we can. You can pay us in pizza", remembered Wentz. As the tour concluded, the general consensus was that Hurley would be the band's new drummer, and the band began to shop around the three songs from the group's unreleased split as a demo to record labels. The band members set their sights on pop-punk labels and attempted, with considerable effort, to join Drive-Thru Records. A showcase for label co-founders went largely mediocre, and the band were offered to sign to side label Rushmore, which they declined. They got particularly far in discussions with The Militia Group and Victory Records, and Bob McLynn of Crush Management became the band's first manager. The band re-entered the studio with O'Keefe to record several more tracks to create label interest. Wentz felt "in the backseat" in writing the songs and temporarily questioned his place in the group, but Stump argued in his favor: "No! That's not fair! Don't leave me with this band! Don't make me kind of like this band and then leave it! That's bullshit!"