"Know Your Enemy" is a song by the American punk rock band Green Day. It was released as the first single from their eighth album 21st Century Breakdown, through Reprise Records on April 16, 2009. Billboard described the song as being lyrically "just as politically charged as last time (on American Idiot)", having a "Modern Rock radio-ready chorus", whilst the song's producer Butch Vig "brings enough punk sheen for mass appeal." It was the first, and one of only three songs, to top the Rock Songs, Alternative Songs and Mainstream Rock Tracks Billboard charts at the same time. Know Your Enemy has been certified Gold by the RIAA and has sold 798,000 copies as of August 2010.

It was referred to by Spin as sounding "like an out-take from the Dookie sessions".About.com's Bill Lamb said the song was "standard issue Green Day", "not a stroke of genius" and "not on par with their last album", but said it was a "great appetizer" for one of the most eagerly awaited albums of 2009.Pitchfork Media described the track as having "pedantic stiffness" and "nothing more than a rote protest song preaching to the converted".Rolling Stone called the song one of the highlights of the album, praising the punk-oriented sound and calling it a "Clash-size bootboy chants".Billboard said that "The lead single from the band's new release, "21st Century Breakdown", finds the group just as politically charged as last time", but also said that "While the subject matter is broad and may be missed by some, "Enemy" has enough charm to amp up the airwaves until (21st Century) Breakdown is released."

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