This was also one of the first songs recorded by the band for which Robert Plant received writing credit. According to rock journalist Stephen Davis, the author of the Led Zeppelin biography Hammer of the Gods: The Led Zeppelin Saga, the lyrics for this song reflect a romance Plant had with his wife’s younger sister. “What Is and What Should Never Be” was performed live at Led Zeppelin concerts between 1969 and 1972 (and played once in 1973). A live version taken from a performance at the Royal Albert Hall in 1970 can be seen on the Led Zeppelin DVD. Another was included on disc two of the live triple album How The West Was Won.
The song inspired the name of an episode of the popular teen drama One Tree Hill, and the name of an episode of the paranormal drama Supernatural, as well as an episode of popular half-hour comedy That 70s Show, and one from the action drama Covert Affairs. Billy Joel also played it as part of the intro to the “We Didn’t Start the Fire” medley on the 1997 VH1 Storytellers episode on his career.Record producer Rick Rubin has remarked, “The descending riff [of “What Is and What Should Never Be”] is amazing: It’s like a bow is being drawn back, and then it releases. The rhythm of the vocals is almost like a rap. It’s insane — one of their most psychedelic songs.”