This thesis explores the prosodic phonology of Middle High German (MHG), aiming to provide a thorough account of its structure as a system and consider its role in accounting for key sound changes. It highlights the pertinacity of certain prosodic structures, despite the sometimes quite substantially different surface output between MHG and Modern Standard German (NHG). Much of the complexity of the NHG phonological system has its roots in the mediaeval period and the high level of language contact at this time, notably with Romance languages (and Old French in particular). I argue that much of the modern phonological system’s surface complexity results from the interaction of large-scale Romance borrowing (invariably with right-edge prominence and final superheavy syllables) and pertinacity within the phonological grammar, particularly in relation to the uneven trochee, Prokosch’s Law and the leftward prosodic incorporation of affixes and clitics. Standard accounts of MHG typically provide a phonemic inventory and traditional grammar, neglecting suprasegmental prosody and rarely providing a detailed treatment of the phonological system as a whole. This research takes a holistic approach, aiming to address this gap and provide a focused discussion of a range of prosodic phenomena. It explores not only segmental quantity, but its relationship to higher prosodic structures, including the syllable, foot and prosodic word. This raises implications for phonological theory more generally—including the role of recursivity within the prosodic hierarchy and the lack of isomorphism between surface syntax and phonological phrasing—and provides new insights into lexical stress and cliticisation. ‘Parzival’, a sprawling grail romance from the early thirteenth century, is central to this analysis; it represents a large corpus of real poetic data from a single source, enabling a closer, more nuanced analysis of a particular synchronic snapshot of MHG (at a time when diphthongisation was present, but open syllable lengthening and degemination were yet to occur).