The evolution of the genitive noun phrase in English has been the subject of numerous studies, yet some aspects of this evolution have received less attention than others. In this study I address two of these less studied aspects: the evolution of the plural genitive noun phrase in Middle English (1150-1500), and the decline of the overtly case-marked genitive modifiers (singular and plural) in the same period. The former has generally been presented as following the same path of the singular genitive noun phrase; the latter has been all but ignored, with only a single study (Thomas 1931) which explicitly examines the use of the genitive definite article and strong adjective. The study uses text samples from two electronic corpora, the Linguistic Atlas of Early Middle English and the Penn-Helsinki Parsed Corpus of Middle English, Second Edition, as well as samples from printed editions. The texts used in the present study have been selected with the aim of covering as wide a geographical and chronological range as possible. The thesis examines how and why the number of endings for the genitive plural inflection first increased (in the period up to about 1350) and then decreased (from 1350 onward), a fluctuation not found in the singular inflected genitive noun. The number of available inflectional endings increased due to the morphophonological weaknesses of the -V ending type – the dominant ending type inherited from OE – leading to instabilities in the inflectional system which allowed alternatives to arise. However, the number of genitive plural inflectional endings then decreased, apparently affected not only by the phonological strength/weakness of the ending types but also the type of noun phrase that these were associated with. The inflectional ending which survives, -Vs, is most commonly found with genitive noun phrases in which the genitive noun is animate and the noun phrase has one of the genitive functions labelled POSSESSIVE in this study. This distribution of the various inflectional endings according to animacy and function is related to the rise of the periphrastic genitive plural noun phrase. The initial preferred environment for the periphrastic genitive construction is noun phrases with those functions which will be referred to as NONPOSSESSIVE. As the inflected genitive becomes increasingly restricted to a single noun phrase type, the periphrastic construction expands, to become the default genitive construction by the end of the period. The thesis examines the decline of overtly case-marked genitive modifiers in Middle English, both adjective and determiners. In general, the trend is that morphologically more conservative texts are more likely to preserve case-marked modifier forms, although some marked forms are more widespread due to the development of fixed expressions. Where case-marked modifiers are maintained, historical grammatical gender agreement and the strong/weak adjective distinction are often preserved. Factors which play a role in the survival of marked modifiers are chronological distribution, impact of Old English exemplars, and the development of certain fixed expressions with the adjectives. Thomas (1931) considered the loss of case-marked definite articles and strong adjectives to be the principal factor in the shift from inflected to periphrastic genitive constructions, but the evidence from the present study shows that this is not the case for all texts.