In this article, I advocate for a syntactical definition of harmonic function in rock music such that function is acquired not by a chord's scale-degree content but by its role in the context of a song's form. In rock songs, the syntactical role of dominant, for example, is often played by chords unrelated to V, such as IV, ii, ♭VII, or even versions of I. A theory of harmonic function rooted in chord category—e.g., ascribing dominant function to any chord related to V—inadequately accounts for rock's harmonic organization. I argue that syntactical elements underlie many existing conceptions of harmonic function, but theories rooted in common-practice repertoire nearly always involve chord category to some degree. Separating syntactical and categorical elements not only leads us to a fuller understanding of rock's harmonic idiom, but also reveals similarities between rock music and common-practice tonal music that many theorists insist do not exist.