Big Bang Theory is the fifteenth studio album and the first covers album by the band Styx, released in 2005. It consists of cover versions of classic rock songs.
In 2004, Styx performed a cover of the Beatles song "I Am the Walrus" at Eric Clapton's Crossroads Festival, where the song was received so well that it was released as a single. The video featured original bassist Chuck Panozzo as the "eggman". The single's success resulted in the band recording this album of cover songs. As a result of an appearance on the Today Show the week of the album's launch, the album reached No. 46 on the Billboard Top 200 Albums chart, Styx's first time in the top 50 since 1983's Kilroy Was Here. However, the album only charted for one week. The single "I Am the Walrus" reached a high of number 27 on the Billboard Heritage Rock Chart, but failed to chart on any other rock or pop chart.