This dissertation offers the first systematic study of the relationship between historiographical theory and literary form. I show how historians embody their conceptions of historical progress in the literary forms of their work. Beginning in the twelfth century, historians embrace a theory of historical continuity, which I call continuous history, in which continuity is seen as simultaneously continuous and discontinuous. Descriptions of catastrophic events such as conquests may rupture the smooth progression of history, but they are ultimately incorporated into the overarching narrative of the text. The resulting text typically registers this discontinuity through formal variation, which similarly challenges but does not destroy the narrative cohesion of the text. This dissertation shows how Middle English verse chroniclers utilize this theory and practice of continuous history in their own vernacular works, up to the mid-fourteenth century. It then traces how English historians from the mid-fourteenth century onwards, driven by changes in dynastic politics and intellectual culture, begin to reject continuous history in favor of new approaches such as universal history, de casibus history, and genealogical history. In contrast, however, late medieval Welsh historians persist in their use of continuous history as their primary historiographical paradigm. As a result, the mid-fourteenth and early fifteenth centuries witness the first divide of the shared British historiographical tradition into distinct English and Welsh traditions. Finally, this dissertation argues that medieval writers conceptualize the compilation of historiographical manuscripts and the writing of historiographical narrative in similar terms. For this reason, the late medieval efflorescence of genealogical rolls can be interpreted as part of a broader trend towards genealogical history in the later Middle Ages.