The Green Day we’ve known for the last 15 years—a band who've admirably held onto their core ’90s pop-punk roots but have since evolved into a much broader, more eclectic outfit than even they would’ve predicted—first fully materialized on Warning, their sixth studio album. Released on October 3, 2000, the record made good on the transition that began with 1997’s Nimrod, taking the fusion of radio pop and three-chord angst from that release and fine-tuning it, to reinvent the band that would later pen the 2004 commercial smash, American Idiot.

It was a good time for the change, too. At the moment of Warning’s release, the younger, hungrier pop-punk bands who’d go on to define the ensuing millennial punk/emo explosion were all dropping their most important albums: Earlier in ’99, Blink-182 had issued their wildly successful and incredibly influential Enema of the State; then, the week before Warning debuted, a raucous gang of South Florida teens called New Found Glory released their game-changing Self-Titled record. A few months later, by the early summer of 2000, this class was joined by Sum 41, who seemingly came out of nowhere from the wilds of Canada with their momentous release, All Killer… No Filler, while New Jersey’s Saves the Day brought emo to the masses with Stay What You Are – an album still adored today. So, for Green Day, it was a great opportunity to try something new – and, by that point, they’d matured enough to actually be able to pull it off.

Thus Warning is an intriguing assortment of tracks – some of which recall classic Dookie-era Green Day while others explore completely unchartered territory for the band (e.g. “Misery”), all executed with the most polished and sonically dazzling veneer of any of their records to that point. And although there are surely some forgettable moments, the album also offers some of the strongest, most widely appreciated tracks of Green Day’s career. It’s certainly not a record to be overlooked.

The title track sets a great tone for the other 11 cuts that follow; “Warning” has lyrics brimming with the snarky defiance of authority that made Green Day punk counterculture heroes in the ’90s. But instead of frontman Billie Joe Armstrong sneering over distorted guitars and blasts of high-speed Tré Cool drums, the backing music surprises with a steady mid-tempo roll of acoustic guitars and walking bass by Mike Dirnt that’s tight in the pocket with Cool’s almost Ringo Starr-esque beats. The track is a perfect union of Green Day’s past, present and future.

That’s not to say to the album doesn’t rock, though. Indeed, most of Warning’s most memorable moments are when the trio cut loose and let ’er rip. but unlike on prior albums, there are very few jagged edges – unless that’s the specific goal. “Church on Sunday” is a terrific blast of roaring chords and frenetic drum rolls, but it’s also a ratchet-tight pop song with gargantuan hooks; ditto “Castaway,” “Waiting” and “Hold On,” which also fit snugly into the Green Day back catalog, but offer more diverse arrangements and sonic flourish than predecessors. Perhaps most important to mention is “Minority” – another rawker, but one that showcases just how far the band had come, merging bitter politpunk over crunching guitars that give way to lively acoustic strumming and, at one point, a harmonica solo.

Unlike on prior Green Day albums, there are very few jagged edges on 'Warning' – unless that’s the specific goal.

Green Day were also unafraid to explore their softer side on Warning, especially after the inescapable widespread airplay of “Good Riddance (Time of Your Life)” from Nimrod. In that vein, Warning closes with the mid-temp acoustic strummer “Macy’s Day Parade,” which kind of sounds like a pop-punk reboot of Ben E. King’s “Stand By Me.” It instantly joined the ranks of the band’s classics. As Armstrong sang the lines “I’m thinking about a brand new hope / The one I’ve never known / And where it goes,” it was easy to imagine he could’ve been talking about the unexpected yet extremely promising new phase his band had just entered. Four years later with American Idiot, Green Day would be right back on top, yet in a much difference place than during the days of Dookie.

Critical response to Warning was favorable overall; the album enjoyed an average critic review score of 72/100 on Metacritic, with reviewers generally praising the band’s maturity and skill at crafting solid pop songs. Commercial success, on the other hand, was not quite as warm and fuzzy. Although the record peaked at No. 4 on the Billboard 200 and was certified gold, it posted the lowest sales dip in Green Day’s career as the band’s first major-label album to not go multi-platinum.

Perhaps the decline in sales had to do with the band’s ’90s-era fans growing up and branching out; and perhaps pop-punk fans were also too obsessed with newer arrivals like Blink, NFG and Sum 41 at the time to care as much. But the album’s retail performance had little to do with the level of quality contained within, and time has since been kind to Warning’s legacy – especially thanks to multiple standout tracks that remain Green Day essentials. Two years after Warning’s debut, Green Day were sharing stages with Blink and Saves the Day on the Pop Disaster Tour, but the album was anything but. In truth, Green Day were all grown up at last, and it still sounded pretty great.

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