This article presents a critique of the commonplace trope that holds genre to have declined in relevance under modernism. Contrary to the widespread notion that composers’ repudiation of received tradition rendered the very idea of genre categories obsolete, this article argues that such categories have never ceased playing a decisive role in the production, circulation, and reception of post-1945 art music. In interrogating the assumptions that underpin the “decline-of-genre” thesis, this article underlines the utility that renewed attention to genre and its framing effects may have for the analysis of this repertoire. To this end, an alternative to standard theories of genre is advanced, one that draws on actor-network theory to destabilize categories too often conceived as fixed, solid, and binding. This revised theory of genre is applied to Gérard Grisey’s six-part cycle, Les espaces acoustiques (1974–85). Habitually regarded as an exemplar of spectral music, Grisey’s cycle may be understood as participating in a number of additional generic contexts at the same time. Taking such generic overdetermination into account not only sheds light on the range of conflicting interpretations that Les espaces acoustiques affords but also suggests how music analysis might better address the heterogeneous contexts and multiple listener competences that this and other musics engage.