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.