This thesis is concerned with the longform poetics of Charles Olson and of John Cage, and with the role indeterminacy plays in their constitution and reception. The work of these authors poses unusual and particular challenges to readers, and it is towards readers and reading that this thesis is primarily oriented. Each chapter describes a problem or difficulty which these texts create for readers, and attempts to model that difficulty as clearly as possible in order to demonstrate how it forces readers to reassess received readerly protocols. As such, the thesis is also concerned with the limits of traditional critical methodologies in the face of such works. Though the concrete examples presented are mostly taken from a relatively circumscribed time and culture – the USA post-World War Two – I claim that the problematics of indeterminacy herein discussed are generally prevalent in long poetic forms, and in a certain sense constitutive of them. The thesis maps how ‘indeterminacy’ as a concept within literary criticism conflicts with that model of criticism concerned primarily with the ‘close reading’ of texts and the hermeneutic elucidation of ‘meaning’ thereby. Between historicism and close reading, it argues that this indeterminacy is most pervasive and yet most critically overlooked within traditions of what I call ‘longform’ poetics. The Introduction, discusses the unfitness of Cage’s early text ‘Indeterminacy’ to traditional modes of close-reading as exemplified in I.A. Richards and William Empson. It then recounts the developing discourse around poetic indeterminacy as it emerged through Roman Ingarden, Wolfgang Iser, Marjorie Perloff and Charles Altieri, and how that discourse increasingly configures the question of indeterminacy less around meaning and more around reading as an activity in itself. Chapter One provides a critical redescription of Olson’s hugely influential manifesto-essay ‘Projective Verse’ via comparison to Muriel Rukeyser’s The Life of Poetry. Chapter Two addresses the problem of reading speed with reference to Olson’s interest in the cinema. Chapter Three describes the poetics of heterogeneity and surprise exemplified by Cage’s Mushroom Book. Chapter Four investigates the arrangement and disarray of Olson’s ‘archive poetics’ and his insistent habit of listing. Chapter Five considers how Cage’s cavilling over the idea of ‘ideas’ informs and deforms his huge mesostic lectures I-VI. Chapter Six uses Olson’s interest in models to tease out the constitution of his longform poetics on a set of indeterminate part-whole relations. Chapter Seven traces the effects of typos in two editions of Cage’s Anarchy, and in the thought and editorial practices of Olson. Throughout, the thesis delineates various protocols for reading, models for how to engage the longform texts of Olson and Cage, aiming to demonstrate how for these poetries one needs to select and ‘read through’ a poetics as a sort of optic, one through which such reticent texts can be made legible. Though ‘indeterminacy’ has a long history both within Cage’s own work and in the subsequent scholarship, this thesis does not simply follow his sense of the term. Rather, in tandem with Olson’s own pervasive though less explicit engagement with the