wannabeanimator:

DreamWorks’ Shrek was first released on May 18th, 2001.

The song “All Star” by Smash Mouth, heard in the opening credits, was only placed in the film for test audiences until a new song could be found. But test audiences loved it, and the producers kept it in. When the producers decided to keep “All Star” they decided to let the band sing the last song in the movie, “I’m a Believer.” (x)