This dissertation examines several late medieval Lives of St. Margaret written in England to show that the monsters of the Life offer both a synchronic and diachronic perspective on the construction of Christian sexual identity in both professional religious and lay communities in medieval England. St. Margaret was one of the most popular saints in medieval England, and monsters play a key role in her martyrdom. Throughout her narrative, Margaret is accosted by a demonic prefect, hungry dragon, and loquacious black demon. Having defeated each monster in turn, she is taken to the place of her martyrdom where she prays for supernatural boons for her adherents. As a virgin martyr, Margaret’s resistance to these monstrous aggressors (and the suffering which she undergoes as a result) is the most important aspect of her story: not only does it represent Margaret’s raison d’être, but also the source of the virtus that benefits her cult. Previous scholarship has focused on Margaret’s resistance to Olibrius as a means to understand her impact on the identities of her virginal or maternal adherents, and on Margaret’s speech and deeds as important socio-cultural data which can be used to inform the context of Margaret’s medieval readers. This dissertation also treats each version in question as a source for information on Margaret’s medieval audience, but rather than concentrating upon Margaret’s speech and actions as previous research has, this dissertation instead focuses on the monsters that populate Margaret’s Life. This focus allows a new evaluation of Margaret’s simultaneous appeal to virgins and mothers through the polysemous figure of the dragon, the didactic elements of the black demon’s speech, the competing claims of religious identity in the figure of Olibrius, and the importance and content of the prayers at the end of Margaret’s Life for her maternal adherents. Equally important is that the diachronic focus of the dissertation reveals that while Margaret herself seems to change little over time – showing a slow metamorphosis from demonic adversary to maternal advocatrix – the monsters are more volatile, changing character as needed to create a narrative that constantly exists in the reader’s present.