Now that the quartet has finally reunited, take a look at what Pete, Patrick, Andy and Joe were doing for the past three and a half years, then see how the reunion unfolded as the band got back together to "Save Rock and Roll" on May 7:
Wentz wasted no time jumping into a new project after Fall Out Boy went on hiatus, forming electronic duo Black Cards with vocalist Bebe Rexha in July 2010. The group's "Dr. Jekyll & Mr. Jive" reached No. 29 on the Dance/Club Play Songs chart in 2011 before album delays occurred, Rexha left the band, and Black Cards added Spencer Peterson to release the "Use Your Disillusion" EP in July 2012.
The Fall Out Boy bassist and ex-wife Ashlee Simpson also divorced during the band's break, and recently he has been working on "Gray," a new book reflecting on his own rise to fame. The novel, co-written by MTV News editor James Montgomery, is set to be released Feb. 19.
"We missed you," Wentz wrote on Twitter, announcing Fall Out Boy's reunion with a link to the band's first new single, "My Songs Know What You Did in the Dark (Light Em Up)" on Feb. 4.
Stump was the only member of the quartet to take on a solo project while Fall Out Boy was on hiatus. His 2011 "Soul Punk" album, which he preceded with the "Truant Wave" EP, spent one week on the Billboard 200 at No. 47, selling 23,000 copies to date according to Nielsen SoundScan.
As a solo artist, the Fall Out Boy vocalist opened for Bruno Mars, Janelle Monae and Panic! at the Disco, collaborated with Lupe Fiasco, and contributed to All Time Low's 2012 "Don't Panic" album. He also married his longtime girlfriend in September 2012.
After denying reunion rumors in October, tweeting "One of these days there may actually be news and you won't be able to hear it over all the rumors," Stump confirmed the reunion on his own Twitter account with a link to Fall Out Boy's official announcement.
Back in 2009, Hurley was the first to worry fans with the group's break, tweeting that they were on "hiatus," before later deleting and re-wording the message.
The Fall Out Boy drummer ventured further into rock after Fall Out Boy went on a break, drumming with multiple bands over the three-year period. He kept up his record label, F-ck City, and drummed for bands like Burning Empires and Enabler. He also formed heavy metal outfit The Damned Things with Fall Out Boy guitarist Joe Trohman, Scott Ian and Rob Caggiano of Anthrax, and Keith Buckley of Every Time I Die.
Guitarist Trohman continued to perform in other rock bands after Fall Out Boy separated, forming The Damned Things with Hurley. The group's 2010 "Ironiclast" album reached No. 1 on Heatseekers Albums and No. 32 on Rock Albums with 31,000 copies sold to date according to Nielsen SoundScan.
The Damned Things is currently on hiatus -- the Anthrax and Every Time I Die members are busy with their other projects -- so Trohman has been working with rock trio With Knives, who released a six-song EP in 2012.
Fall Out Boy's Reunion: A Timeline
October 2012: A friend of the band tweeted that Fall Out Boy was in the process of making new music. The tweet went viral within alternative music circuits, and each member of the band was quick to deny any chance of a reunion.
January 2013: The noise died down for a few months, until January, when blogs started posting that Fall Out Boy was back and an announcement was coming soon.
February 2013: During the first week of February, DirectTV listings revealed that Fall Out Boy was scheduled to be a guest on Jimmy Kimmel's talk show on Feb. 13. All four band members were still staying silent or denying the rumors, keeping their secret until the official announcement hit the web.
The group tweeted out a new photo taken on the morning of Feb. 4 -- showing Pete, Patrick, Joe and Andy throwing old Fall Out Boy records on a bonfire in Chicago. The quartet also released a new single, "My Songs Know What You Did In The Dark (Light Em Up)," announced an upcoming tour, and revealed the title of Fall Out Boy's first album in more than four years, "Save Rock and Roll." Read the full story here.
While the Kimmel performance is still just a rumor, it has been announced that the band will perform at this summer's Reading and Leeds Festivals in England. It has also been suspected, although not confirmed by anyone involved with the band or the event, that Fall Out Boy will perform at the Skate and Surf Festival on May 18-19 in New Jersey.
The band's "Save Rock and Roll" tour kicks off May 14 in Milwaukee, with dates across the U.S. through the end of June. To celebrate the reunion, Fall Out Boy will be playing intimate dates in Chicago, New York and Los Angeles this week.