This thesis argues that case variation in English conjunction is best analysed as being the result of underspecifcation in the morphosyntactic features of lexical items(LIs). This supports the argument made by Adger and Smith (2005, 2010), Adger(2006), Biberauer and Richards (2006), Biberauer and Roberts (2005) inter alia: namely, that morphosyntactic variation does not require any variation-specific mechanisms and can be explained within a Minimalist framework by recourse to either morphosyntactic features or PF-based operations. An examination of the existing studies of case variation in conjunction shows that there is little consensus regarding which case forms are grammatical and the precise nature of the attested variation: whilst some assume that only acc+acc combinations are grammatical, others claim that all combinations are possible regardless of their syntactic position; and the proposals based on the possibility of intra-speaker variation are contradicted by those which assume that only inter-peaker variation exists. A new data set is collected in order to resolve this empirical uncertainty. It shows that nom+nom, nom+acc and acc+acc combinations are grammatical in subject position, but that only acc+acc combinations can be generated in object position. Furthermore, both inter- and intra-speaker variation is attested, with some speakers accepting all three subject-position variants and others accepting only one or two. Having shown that none of the existing analyses can satisfactorily account for both how all variants are generated and for the presence of inter- and intra-speaker variation, I develop an alternative using optional feature underspecification (Adger 2006) to show that all variants can be generated within a single grammar (thereby accounting for the intra-speaker variation) and that this grammar can be restricted to account for the attested inter-speaker variation. The contribution made by this thesis to our overall linguistic knowledge is three-fold. Firstly, it robustly establishes the pattern of attested case forms in English conjunction and demonstrates that both inter- and intra-speaker variation can be observed. Secondly, I identify the mechanisms of Case and feature assignment/agreement within conjoined phrases, and finally, I show how the Minimalist Program can accommodate both inter- and intra-speaker variation within the existing constraints of the programme.