Nerf Herder:  Parrie Gripp, Charlie Dennis, Steve Sherlock, and Linus of Hollywood Joss Whedon, dissatisfied with the theme for Buffy written by whoever had been chosen to write it, was encouraged by Alyson Hannigan to listen to the music of Nerf Herder. Subsequently, Nerf Herder was asked to write a theme for the show and they did. There has been a lot of discussion as to whether they may have "borrowed" a chunk of the melody from Codo, a song by a German band, Deutsch-Ostereichisches-Feingefühl.

Much has been made of the fact that Codo's lyrics mention an alien and UFO's.

It's still a stretch to think that the Buffy Theme might have been grown out of the few bars of Codo that bear some similarity.

In 1999 Nerf Herder had this to say about the writing of the Buffy Theme:

"We did the theme song before the show was on the air. It was a song that had already been written, but we really didn't know where to take it. I had some ideas - it was originally going to have some kind of science-fiction theme, which is weird, because we don't do songs like that. I remember having the title of 'Outer Space Rock' or something like that; that was a working thing."

Below is a version of Codo by DÖF. Below that is the Buffy Theme.