Green Day is an American punk rock trio formed in 1987. The band has consisted of lead vocalist and guitarist Billie Joe Armstrong, bassist and backing vocalist Mike Dirnt, and drummer Tré Cool for the majority of its existence. The band is credited as one of the three main bands, along with The Offspring and Rancid, who put the punk revival into process.

Green Day was originally part of the punk scene at 924 Gilman Street in Berkeley, California. The band's early releases for independent record label Lookout! Records earned it a grassroots fanbase. Nevertheless, its major label debut Dookie (1994) became a breakout success and eventually sold over 10 million copies in the U.S. and 15 million worldwide. As a result, Green Day was widely credited, alongside fellow California punk bands The Offspring and Rancid, with reviving mainstream interest in and popularizing punk rock in the United States. Green Day's three follow-up albums, Insomniac, Nimrod and Warning did not achieve the massive success of Dookie, but they were still successful, reaching double platinum, double platinum, and gold status respectively. Green Day's 2004 rock opera American Idiot reignited the band's popularity with a younger generation, selling five million copies in the U.S. The band's eighth studio album, 21st Century Breakdown, was released on May 15, 2009.

Green Day has sold over 65 million records worldwide and 24.5 million in the US alone. They have won four Grammy Awards; Best Alternative Album for Dookie, Best Rock Album for American Idiot, Record of the Year for "Boulevard of Broken Dreams", and Best Rock Album for the second time for 21st Century Breakdown.

Green Day's sound is often compared to first wave punk bands such as the Ramones, The Clash, Sex Pistols, Stiff Little Fingers, The Jam, and the Buzzcocks. The majority of their song catalog is composed of overdriven guitar, fast, manic drums, and relatively high-treble bass. Most of their songs are fast-paced and under four minutes. Billie Joe Armstrong has mentioned that some of his biggest influences are seminal alternative rock bands Hüsker Dü and The Replacements, and that their influence is particularly noted in the band's chord changes in songs. In fact, Green Day has covered Hüsker Dü's "Don't Want to Know If You Are Lonely" as a B-side for the "Warning" single, and the character "Mr. Whirly" in their song "Misery" is a reference to the Replacements song of the same name. Among other influences, Green Day have also cited Queen, proto-punks The Who, and power pop pioneers Cheap Trick. Armstrong's lyrics commonly describe alienation, ("Jesus of Suburbia", "Boulevard of Broken Dreams", "Road to Acceptance", "Disappearing Boy", "Castaway") hysteria ("Basket Case", "Panic Song", "American Eulogy"), girls ("She", "80" "Only of You","Maria" "She's a Rebel"), growing up ("Longview" and "Welcome to Paradise"), and the effects of doing drugs ("Geek Stink Breath", "Green Day", "Give Me Novacaine"). The Ramones had similar lyrical themes such as hysteria ("Anxiety", "Psycho Therapy"), alienation ("Outsider", "Something To Believe In"), girls ("I Wanna Be Your Boyfriend", "Sheena Is a Punk Rocker"), and drugs ("Now I Wanna Sniff Some Glue", "Chinese Rocks"). Green Day has covered Ramones songs several times, including recording "Outsider" for the tribute album We're a Happy Family, and performing "Blitzkrieg Bop" and "Teenage Lobotomy" when the Ramones were inducted into the Rock and Roll Hall of Fame in 2002.

In reaction to both the style of music and the background of the band, John Lydon, former front man of the 1970s punk band the Sex Pistols commented:

    So there we are fending off all that and it pisses me off that years later a wank outfit like Green Day hop in and nick all that and attach it to themselves. They didn't earn their wings to do that and if they were true punk they wouldn't look anything like they do.

English rock musician Noel Gallagher of Oasis also complained about the band semi-jokingly, claiming that they ripped off his song "Wonderwall" with their song "Boulevard of Broken Dreams". Mashup DJs Party Ben and team9 would later release a mashup of the two songs called "Boulevard of Broken Songs" under the spoonerism Dean Gray.

Cool mentioned in a July 2009 interview that while Armstrong is the primary songwriter, he looks to the other band members for organizational help.

Ever since 1991, some members of the band have branched out past Green Day and have started other projects with other musicians. Notable related projects of Green Day include Billie Joe Armstrong's Pinhead Gunpowder (which also features Green Day's live backup guitarist Jason White), The Frustrators in which Mike Dirnt plays bass, and The Network, in which all three members of Green Day play under fake stage names. Billie Joe Armstrong has also confirmed that the main members of Green Day are in the band Foxboro Hot Tubs. A Foxboro Hot Tubs album titled Stop Drop and Roll!!! was released on May 20, 2008.

In September 2006, Green Day teamed up with U2 and producer Rick Rubin to record a cover of the song "The Saints Are Coming", originally recorded by The Skids, with an accompanying video. The song is to benefit Music Rising, an organization to help raise money for musicians' instruments lost during Hurricane Katrina, and to bring awareness on the eve of the one year anniversary of the disaster.

In December 2006, Green Day and NRDC opened a web site in partnership to raise awareness on America's dependency on oil.

Green Day released a cover of the John Lennon song "Working Class Hero", that was featured on the album Instant Karma: The Amnesty International Campaign to Save Darfur. The band performed the song on the season finale of American Idol. The song was nominated for a Grammy in 2008, but lost to The White Stripes' "Icky Thump".

That summer, the band appeared in a cameo role in The Simpsons Movie, where they perform the show's theme song. Their version was released as a single on July 24, 2007.

In 2009, the band collaborated with theatre director Michael Mayer to adapt their rock opera American Idiot into a one-act stage musical that premiered at the Berkeley Rep on September 15, 2009.

The show then moved to Broadway on April 20, 2010. The reviews have been positive to mixed. Charles Isherwood of The New York Times wrote an enthusiastic review for the Broadway production. He called the show "a pulsating portrait of wasted youth that invokes all the standard genre conventions ... only to transcend them through the power of its music and the artistry of its execution, the show is as invigorating and ultimately as moving as anything I’ve seen on Broadway this season. Or maybe for a few seasons past." Jed Gottleib of the Boston Herald enjoyed the premise of the show but found that "the music and message suffer in a setting where the audience is politely, soberly seated". Michael Kuchiwara of the Associated Press found the show to be "visually striking musically adventurous", but noted that "the show has the barest wisp of a story and minimal character development". Paul Kolnik in USA Today enjoyed the contradiction that Green Day's "massively popular, starkly disenchanted album ... would be the feel-good musical of the season". Time magazine's Richard Zoglin opined that the score "is as pure a specimen of contemporary punk rock as Broadway has yet encountered there's enough variety. ... Where the show fall short is as a fully developed narrative." He concluded that "American Idiot, despite its earnest huffing and puffing, remains little more than an annotated rock concert. ... Still, deserves at least two cheers – for its irresistible musical energy and for opening fresh vistas for that odd couple, rock and Broadway." Peter Travers from Rolling Stone, in his review of American Idiot, wrote "Though American Idiot carries echoes of such rock musicals as Tommy, Hair, Rent and Spring Awakening, it cuts its own path to the heart. You won’t know what hit you. American Idiot knows no limits — it's a global knockout."

The musical has been nominated for several Tony Awards, including Best Musical and Best Scenic Design. It was also nominated for a number of Drama Desk Awards and Outer Critics Circle Awards.

In October, a Green Day art project was exhibited at StolenSpace Gallery in London. The exhibition showed artworks created for each of the songs on 21st Century Breakdown, was supported by the band, and led by their manager Pat Magnarella. He explained in an interview that "basically like rock bands. Most are creating their art, but don't know how to promote it." For Billie Joe Armstrong, "Many of the artists... show their work on the street, and we feel a strong connection to that type of creative expression.”