This essay returns to part 2, chapter 4 of Louis Althusser and Étienne Balibar’s landmark text Reading Capital to interrogate some of the historiographical concepts upon which literary studies continues to rely, especially that of the period. Althusser’s profound critique of the period remains cogent, this article argues, for a field in which individual events and texts and large-scale social and political formations are often bound together by no framework other than their putative belonging to one and the same block of time—that is, a period. Aiming to reform this concept in order to institute a more complex historiography, Althusser invents a version of what came to be known simply as “theory.” (Théorie was the name of Althusser’s series at Maspero, in which Reading Capital and many of his subsequent works appeared.) Thus, while this essay, on the one hand, discloses the limits of Althusser’s own practice of theory (in its ambivalent relation to structuralism), it concludes, on the other, that no genuine reworking is possible of the historical horizon we all presuppose, apart from the revival of theory or one of its avatars or successors. As in Althusser’s example, theory’s task, as yet unfulfilled, is to recast thought and knowledge’s relation to time and history.