If science fiction is a genre, then attempts to think about the nature of science fiction will be affected by one’s understanding of what genres are. I shall examine two approaches to genre, one dominant but inadequate, the other better, but only occasionally making itself seen. I shall then discuss several important, interrelated issues, focusing particularly on science fiction : what it is for a work to belong to a genre, the semantics of genre names, the validity of attempts to (...) define genres, and the connections between genre and normativity. One important but neglected clue to the nature of genres lies in the kinds of disagreements they generate over the assignment of works to genres. I conclude by explaining why these disagreements tell us something about the nature of genres, and discussing in some detail two famous cases of disagreement about whether some work or works are science fiction. (shrink)