The title of the film (as well as the novel upon which it was based) refers to the fact that, according to myth, vampires must be invited in before they can enter someone's home (this is shown in the film when Eli asks Oskar to invite her into his apartment). The English-translated title of the book and film, "Let The Right One In," is taken from lyrics to the song "Let the Right One Slip In" by Morrissey.