Warning: is the sixth full length album by Green Day, released in 2000. Building upon predecessor Nimrod, Warning eschews their trademark punk rock sound with a more folk punk effort. Warning was also the first Green Day album that was not produced by Rob Cavallo since Kerplunk.

Although Warning: reached #4 on the charts and went gold, the album clearly displayed the fact that Green Day was not enjoying the success they had earned in the past, and held a less prominent spot in the mainstream. Despite its high critical response, it represented the lowest commercial slump in the band's career since their 1994 breakthrough Dookie. Many speculated that Green Day had painted themselves into a corner and would be unable to return to punk music. Nonetheless, many fans hold this album in high regard as being creative, experimental, and overall musically successful.

Reaching #4 in the U.S. and UK, Warning: has sold 1 million units in the U.S. and 4 million worldwide as of August 2006.