This dissertation analyzes Chaucer's narrative endings as the recurrent key battlegrounds in the poet's long reception history, whether in the early English manuscript tradition, in the imitative Chaucerian writing of sixteenth-century Scotland, or in modernity's institutional scholarship and continuing tradition of creative response to the poet's works. Combining narratological and historicist perspectives, it first situates Chaucer's endings within what we can reconstruct of medieval narrative theory, and then examines how and why so many later authors rewrite or otherwise intervene in Chaucer's endings, always interrogating the ideological stakes of the literary ending and its place in interpretation. The project contributes to the ongoing recovery of a medieval literary theory by identifying narratological conceptions of the function and purpose of ending in the ars poetica tradition, particularly as expressed by disciples of Horace such as Geoffrey of Vinsauf, Matthew of Vendôme, and John of Garland. Rhetorical handbooks emphasize formal closural devices like prayers, blessings, curses, proverbs, morals, and so on, the same formulas that signal the ends of the majority of Chaucer's narratives but that have received little attention in themselves. I argue that these endings demonstrate Chaucer's adherence to a Horatian rather than Aristotelian paradigm of ending, which privileges the harmonious congruence of parts over teleology and necessity. By contrast, the reception history of Chaucer's endings witnesses a resurgence of neo-Aristotelian reading strategies that center the ending as the culmination and fulfillment of a work.The reception history of Chaucer's Retraction provides the clearest example of the way in which often unarticulated theories of ending have driven response to Chaucer's works. By studying the diversity of methods by which Chaucer's first scribes rubricate and represent the Retraction, we can find evidence for an early understanding of Chaucer's most controversial ending in Horatian terms, as for example in the probable allusion to John of Garland's concept of authorial license present in London, British Library MS Lansdowne 851. As the Retraction enters, exits, and reenters the print tradition, readers employ different theories of ending to render the document more or less central to Chaucer's works, or even to decenter or delegitimate it entirely: Chaucer's endings must always be contextualized within theories of ending both medieval and modern. By attending to the complexity of both medieval literary theory and medieval narrative itself, overall this project builds a case for a more historically responsible narratology of medieval texts, as well as a more expansive one that would consider the implications of the reception history of these texts and the multiple forms that they take. For instance, the intricate structure of the narrative discourse in Chaucer's dream visions becomes further complicated by the existence of multiple versions of these texts in the mouvance or 'hypertextuality' of manuscript and early print culture. The medieval ending is a fundamentally multiplex phenomenon that only grows more so over its reception history.Finally, we must keep in mind that Chaucerian endings were not always written by Chaucer. William Caxton infamously attaches his own conclusion to the unfinished House of Fame, but nowhere is the desire to conclude Chaucer with an ending of one's own so pronounced as in late medieval Scotland, where I argue that an aesthetics -- and politics -- of the 'supplementum' dominates. I do not exclusively consider the works of the so-called 'Scottish Chaucerians,' but also examine more obscure attempts to close Chaucer in Scotland, including the unique ending to the Parliament of Fowls that appears in Bodleian Library MS Arch. Selden. B. 24. The result in this manuscript, as in many Scottish poems of the period, is a 'Scotticization' of the Chaucer tradition effected through rewritings of the poet's endings. The supplementum claims the works of another author for oneself and one's own tradition: through the mediation of his endings, Chaucer becomes the property of the Scots, just as, in a more diffuse way, some of Chaucer's endings go on to become the common property of much later literary traditions. Thus, the project concludes with an exercise in applying the fusion of information and evolutionary science known as 'memetics' to familiar problems of imitation and influence in literary history: we cannot dismiss the repetition of formulaic endings encouraged by a Horatian poetics of ending -- such as the 'Go, litel bok' topos -- as simply a primitive feature of medieval literature. This project's thorough examination of Chaucer's endings and their legacies, medieval to postmodern, will also shed light on the proliferations of formulae and theories of ending that continue in the present.