In a 2010 essay on the fate of New Historicism, Steven Justice proposes that critics should rethink methods of contextualizing poetry within history. As Justice explains, New Historicism has lead critics to construct increasingly local, microscopic, discrete contexts for reading poetry. How, he asks, might we account for the moment in which poetry is written without isolating that context from larger processes of time and change? This dissertation argues that Chaucer's poetic form can itself point the way towards a dynamic understanding of the moment in which poetry is composed. Chaucer writes in a developing vernacular defined through its constant tendency to change. Meanwhile, he builds his works using borrowed material from old texts: sources both Latin and vernacular, classical and medieval. In order to understand the significance of this layered literary form, I compare Chaucer's poetry to the textual culture of Trecento Italy. There, the growing popularity of Roman history among vernacular readers inspired scribes and translators to develop sophisticated methods of using form to reflect historical, lexical, and cultural difference between past and present. Meanwhile, Trecento Italian poets recognized that the vernacular itself could provide an important site for theorizing historical alterity. As Dante argues, given enough time to develop, a vernacular language will appear completely foreign to its original speakers. Using Italian texts as comparative context for close readings of Chaucer's poetry, I find that Chaucer is also interested in how language and literature change over time. However, rather than focusing on the difference between ancient poetry and medieval texts, Chaucer hones in on the propensity for alterity within his own poetic form and lexicon, which is amplified by the influence of translated and adapted texts. His poetry represents its own moment as being in a state of transformation driven, in part, by the very act of writing.