Digression is widely considered a mark of disordered or evasive discourse. Modern legal, philosophical, and political writing largely disavows this trope, regarding it as a departure from the model of rational exposition institutionalized under the Enlightenment. And yet, as the rhetorical figure of digression has grown increasingly marginalized within the decorum of public discourse, it has come to occupy a central position in the private discursive world of poetry. Changing Subjects outlines an anatomy of 'the excursus' within twentieth-century American poetics; moving from aesthetics to the archive to narratology to theories of identity, this study considers the various spheres in which American writers of the period revise prior models of purposeful discourse by cultivating a poetics of digression in the modern poem. The opening section considers the manner in which Wallace Stevens employs digression within the ars poetica genre to deconstruct aesthetic theory under High Modernism; the second chapter examines Marianne Moore's use of the excursus to organize archival knowledge in the Progressive poetry of instruction; the third section turns to Lyn Hejinian's construction of a digressive narratology intended to unsettle master-narratives of the Cold War era; the fourth chapter treats digression as a strategy for fashioning the self in the poetry of Walt Whitman and Frank O'Hara; and the book concludes with a survey of "Elliptical" strategies employed by a new generation of poets, writing in the wake of John Ashbery's aleatory craft, who seek to extend the digressive project of American poetry into the 21st Century.