[1] Building upon its predecessor Nimrod (1997), it eschewed the band's trademark punk rock sound and incorporated acoustic elements and pop and folk styles.

Although it peaked at number four on the US Billboard 200, Warning represented the lowest commercial slump in Green Day's career, being their first album since signing to a major label not to achieve multi-platinum status.

924 Gilman Street, the punk club in the band's hometown that had once banned Green Day after the group signed with a major label, booked bassist Mike Dirnt's side project the Frustrators for a show.

"[9] For Warning, Green Day initially opted to work with a producer other than Rob Cavallo, who had handled the production of the band's previous three albums.

[11] The title track, a "densely produced blast of layered vocals [and] strummed acoustic guitars", features a "circling bass riff" similar to that of "Picture Book" by the Kinks.

"[21] According to Sal Cinquemani of Slant Magazine, the lyrics of "Minority" serve as "a reminder of the youthful mentality of Green Day's early work".

[23] According to some publications, such as LA Times, The Buffalo News, Sun-Sentinel, and Music Box Magazine Green Day departs from their punk rock sound with this album.

[54] Entertainment Weekly's Ken Tucker perceived a maturity in the album's lyrical content and called its music "as peppy as any Green Day have recorded".

[14] Charlotte Robinson of PopMatters commended Billie Joe Armstrong's lyrics and noted the band for embracing "the pop bent that has always been a part of their sound".

[12] Los Angeles Times writer Natalie Nichols wrote that the album "reveal[s] them shaking off the transitional aspects of 1997's 'Nimrod' to craft a more coherent, less aggressive but still rebellious collection that also draws on the even older pop traditions of Bob Dylan, the Beatles and the Who".

[56] "Metal" Mike Saunders of The Village Voice viewed Warning as the band's best work and compared its music to that of the Beatles' Rubber Soul (1965).

[61] In his consumer guide for The Village Voice, critic Robert Christgau gave the album an A− rating,[59] indicating "the kind of garden-variety good record that is the great luxury of musical micromarketing and overproduction.

[62] Christgau noted "professionalism, craft, artistic growth" rather than maturity in Armstrong's songwriting and elaborated on his change in musical direction, stating: He's abandoning the first person.

[18] Greg Kot of Rolling Stone wrote that Armstrong "can't muster the same excitement for his more mature themes" and stated "Who wants to listen to songs of faith, hope and social commentary from what used to be snot-core's biggest-selling band?".

[58] Adam Downer of Sputnikmusic gave it three out of five stars and commented that it "consists of instant classics like Minority and Macy's Day Parade, but it also is filled with garbage songs as well".

Nor does everything in the stylistic grab bag fit", but concluded by complimenting Armstrong's "earnestly good-hearted" lyrics and wrote that "this album is after... evidence that even the snottiest deserve grace and the chance to age into warmth".

Music called the album "crafty pop-rock" and stated "Some might wish Green Day never decided to grow up like this, but others might consider it a starting point to take the band seriously".

[15] Writing in 2009 with regard to Warning's lackluster commercial performance, James Montgomery of MTV News called the record "unjustly overlooked" and applauded Armstrong's "super strong" songwriting on the album.

[43] Catucci called the songs "speedy, neatly packaged reinterpretations of pop-rock history, from the Beatles to Creedence Clearwater Revival to the Ramones themselves".

[43] AllMusic editor Stephen Thomas Erlewine called it "gleeful, unabashed fun" and complimented Green Day for "embracing their fondness for pop and making the best damn album they'd ever made".

[55] Dom Passantino of Stylus Magazine cited it as "the most influential album on the British pop landscape since 1996 (Spice, naturally)", noting it as a significant influence on "the two biggest bands in the UK at the moment, and indeed for the past few years, Busted and McFly".

[65] Passantino called Warning "a great album" and viewed that Green Day "seemed to be bored with their genre-medium, but simultaneously knowledgeable that any attempt to boundary-hop will end with them falling on their face".

[65] While Green Day was nearing completion of Warning, the band announced it would be performing on the 2000 Vans Warped Tour during the summer before the album's October release.

[13][17] Green Day also co-headlined a "shared bill" with fellow Californian pop-punk band Blink-182 on the Pop Disaster Tour from April to June 2002.