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In popular music, a side project is a project undertaken by one or more persons already known for their involvement in another band. It can also be an artist or a band temporarily switching to a different style.
Usually these projects emphasize a different aspect of that person's or that band's musical interests that they feel they cannot explore within the boundaries established by their main project. Side projects can later become full-time endeavours, but should not be confused with quitting a band for a solo career or another band.
One example of musical side projects is Kiss's decision in 1978 to have each member of the band, Gene Simmons, Paul Stanley, Ace Frehley and Peter Criss, simultaneously release solo albums. In 1992, The Melvins released solo EPs in a similar fashion.
"Side project" may also refer to pursuits of famous individuals outside of their primary fields. For instance, Wicked Wisdom is a "side project" of actress Jada Pinkett Smith. Side projects often occur at a crossroads of a celebrity's career.
A side project can also be a band that takes on an "alter-ego", usually in order to play a different style of music that their fans are not used to. An example of this type of side project would be the band Weezer, who is also the Nirvana cover band Goat Punishment.
Another example of a side project is when in 1990, Jesper Stromblad of Ceremonial Oath decided to start a side project known as In Flames because he wanted to start a new project that combined Iron Maiden and Death Metal all together which he said he had never heard of before, the side project then grew into a band in 1995 and is now one of the most influential Metal bands of all time, and still together without Jesper.
Famous or notable side projects 
- (a+) machines, a side project of Matt Noveskey of Blue October.
- The Almost, a side project of Aaron Gillespie of underOATH.
- Alter Bridge, a side project of Mark Tremonti, Brian Marshall, and Scott Phillips of Creed.
- Anchor & Braille, a side project of Stephen Christian of Anberlin, Aaron Marsh and Copeland
- A Perfect Circle, a side project of Maynard James Keenan of Tool.
- The Art Goblins, a side project of Eddie Argos and Jasper Future of Art Brut.
- Blaqk Audio, a side project of Davey Havok and Jade Puget of AFI
- Box Car Racer, a side project of Tom DeLonge and Travis Barker (then of Blink-182), and David Kennedy of Hazen Street.
- Brad, a side project of Pearl Jam and Satchel.
- The Breeders, a side project of Pixies and Throwing Muses.
- Cobra Starship, a side project of Gabe Saporta of Midtown.
- City and Colour, a side project of Dallas Green.
- The Cross, a side project of Roger Taylor of Queen.
- Dashboard Confessional, a side project of Chris Carrabba of Further Seems Forever
- Dead by Sunrise, a side project of Chester Bennington of Linkin Park and all members of Julien-K
- Eagles of Death Metal, a side project of Josh Homme of Queens of the Stone Age.
- Fever Ray, a side project of Karin Dreijer Andersson of The Knife.
- Fight the Sky, a side project of Jacoby Shaddix of Papa Roach.
- The Fireman, a side project of Paul McCartney.
- FM Static, a side project of Trevor McNevan and Steve Augustine of Thousand Foot Krutch.
- Fort Minor, a side project of Mike Shinoda of Linkin Park.
- Foxboro Hot Tubs, a side project of Green Day.
- The Glove, a side project of Robert Smith and Siouxsie and the Banshees' Steven Severin of The Cure.
- Goon Moon, a side project of Jeordie White of Marilyn Manson.
- Gorillaz, a virtual band composed of four fictional animated band members, created by Damon Albarn from Blur, and Jamie Hewlett, the co-creator of the comic book Tank Girl.
- Gov't Mule, a side project of Warren Haynes and Allen Woody of the Allman Brothers Band.
- I Can Make a Mess Like Nobody's Business and Ace Enders and a Million Different People fronted by Ace Enders, formerly of The Early November
- Jack's Mannequin began as a side project of Andrew McMahon of Something Corporate
- Jonathan Davis and the SFA, a side project of Jonathan Davis of Korn.
- Liquid Tension Experiment, a side project of Dream Theater featuring John Petrucci, Mike Portnoy, Jordan Rudess, and progressive rock icon bassist Tony Levin.
- MD.45, a side project of Dave Mustaine and Jimmy DeGrasso from Megadeth with Lee Ving from Fear
- Mike + The Mechanics, a side project of Mike Rutherford of Genesis.
- Mrs. Scabtree, a side project between Marilyn Manson, Jeordie White and Jessicka from Jack Off Jill.
- The Network, supposedly a side project of all the members of Green Day.
- The New Amsterdams began as a side project of Matthew Pryor of The Get Up Kids
- Nick Jonas & the Administration, a side project of Nick Jonas of the Jonas Brothers
- The Nightwatchman, the alter ego of Rage Against the Machine and Audioslave guitarist Tom Morello.
- The Operation M.D., a side project of Juliette and the Licks guitarist Todd Morse and Sum 41 bassist Cone McCaslin.
- Pinhead Gunpowder, a side project of Billie Joe Armstrong.
- Planet Piss, a side project of William Murderface of Dethklok.
- The Postal Service, a side project featuring Ben Gibbard of Death Cab for Cutie and Jimmy Tamborello of Dntel.
- Probot, a Dave Grohl side project featuring various 70s and 80s heavy metal vocalists.
- ProjeKct X, the side project of King Crimson.
- Prozzäk, a side project with James McCollum and Jason Levine who are two members of The Philosopher Kings.
- Puscifer, another side project consisting and organized by Maynard James Keenan of Tool.
- The Raconteurs, consisting of Jack White (from the White Stripes), Brendan Benson, Patrick Keeler (from The Greenhornes), and Jack Lawrence (from The Greenhornes and Blanche).
- Satan on Fire a side project between Jeordie White and Brian Warner.
- Sixx:A.M., a side project of Nikki Sixx of Mötley Crüe.
- She Ilence, a side project of Bernardo Risquez and Michael Langeder.
- Some Devil, a side project of Dave Matthews from the Dave Matthews Band.
- Some Girls, a supergroup/side project consisting of current members of The Locust and The Plot to Blow Up the Eiffel Tower, and ex-members of Give Up the Ghost (formerly American Nightmare).
- The Tear Garden, a project consisting primarily of Edward Ka-Spel from The Legendary Pink Dots and Cevin Key from Skinny Puppy.
- The Sound of Animals Fighting, a side project consisting of Rich Balling, Matt Embree, and Christopher Tsagakis of Rx Bandits, Anthony Green of Circa Survive, Derek Dougherty and Randy Strohmeyer of Finch, Marc McKnight of Atreyu, Rich Zahniser of Never Heard Of It, Matthew Kelly of The Autumns, Craig Owens of Chiodos, and Keith Goodwin of Days Away
- Temple of the Dog, featuring members of Soundgarden with Pearl Jam and the start of that band.
- Thee Silver Mt. Zion Memorial Orchestra & Tra-La-La Band, originally a side project of Montreal post-rock band Godspeed You! Black Emperor.
- Them Crooked Vultures, a side project feautring Dave Grohl (Foo Fighters and Nirvana), Josh Homme (Queens of the Stone Age) and John Paul Jones (Led Zeppelin).
- Three Fish, featuring Jeff Ament of Pearl Jam.
- Tom Tom Club, a side project of Tina Weymouth and Chris Frantz of Talking Heads.
- White Hot Odyssey, a hard rock side project featuring vocalist Steve Perry and guitarist Jason Moss of the Cherry Poppin' Daddies.
- The Whitest Boy Alive, a side project of Erlend Øye of Kings of Convenience