On May 11, 2006, Dave Baksh announced in a statement through his management company that he was leaving Sum 41 to work with his new band, . Baksh's departure from the band was said to have been due to his desire to play music with a more classical metal sound.

On  Deryck made the first official comment on the subject on the official message board post addressing Dave's departure. Deryck confirmed Dave's departure and announced that the band would only be replacing Dave with a touring guitarist, who wouldn't be in videos, photo shoots, albums, or have any decision-making power in the band.

Sum 41's Song, So Long, Goodbye is to Dave's departure. Dave Will Be in Some Pictures in This Site, But, He is no Longer a Member.

Underclass Hero is Sum 41's fifth overall studio album and the first to be released without former guitarist Dave Baksh. The album was first released July 18, 2007 in Japan. It was released under the Island Records label and distributed worldwide by Universal Records, and in Canada by Aquarius Records. The title's similarity to John Lennon's hit "Working Class Hero" is not a coincidence, according to the band's front man Deryck Whibley, who in an interview with Sun Media claimed Lennon as his favorite songwriter. Also, Green Day released their cover version of Lennon's song Working Class Hero only a month or so before Sum 41's album was released.

Underclass Hero is over ten minutes longer than any previous Sum 41 album. This is a result of there being a total of fourteen tracks (plus one hidden track) compared to the set of twelve or thirteen the band has used before. Mostly however, the length is due to a majority of the songs being much longer than the usual three minutes existing on preceding full-lengths.

"I had to decide what I wanted to say with my music," explains Whibley. "I asked myself all these questions and then just pulled up my own answers and started writing songs based on those themes. I wanted to make an album that meant something important from beginning to end. I wanted it to have relevance and significance. It's not a concept album It's not about fictional characters in a made-up story but there is a constant idea that runs through the record. It's a deeply personal statement that reflects the confusion and frustration in modern society." In addition to this, Cone and Stevo have begun to take a bigger role as writers and one song, King of Contradiction, was written solely by the two as well a two songs written by Stevo alone, and are therefore the first Sum 41 album tracks (minus a handful of Pain for Pleasure songs) not to be at least co-written by Deryck.

The album has spawned two singles, "Underclass Hero" and "Walking Disaster". Another song called "March of the Dogs" has also been released as an album preview. On June 26, at 5PM eastern, Walking Disaster was played on the Buzz 103. It has since leaked onto the internet and has become the second single from the album. On July 6 2007 Walking Disaster was added to Sum 41's MySpace playlist. 'Speak of the Devil' and 'Count Your Last Blessings' were leaked onto the internet on July 13 2007. The entire CD was leaked onto file-sharing networks on July 14 2007.