"Know Your Enemy" is a protest song by American rock band Green Day. It is the third track on their eighth album, 21st Century Breakdown, and it was released as the lead single through Reprise Records on April 16, 2009, and the group's first single since "Jesus of Suburbia", released 4 years earlier. Billboard described the song as being lyrically "just as politically charged as last time ", 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.

21st Century Breakdown is the eighth studio album by the American rock band Green Day, released on May 15, 2009, through Reprise Records. Green Day commenced work on the record in January 2006 and forty-five songs were written by vocalist and guitarist Billie Joe Armstrong by October 2007, but the band members did not enter studio work until January 2008.