Warning: is the sixth full length album by Green Day. Building upon predecessor nimrod., Warning: eschews their trademark punk rock sound for a predominantly acoustic effort. Warning: was also the first album not being produced by Rob Cavallo since Kerplunk!. 

Although Warning: reached #4 and went gold, the album clearly displayed the fact that Green Day were 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.

Source: Wikipedia