This thesis argues for a minimalist theory of dual lexicalization. It presents a unified analysis of South Slavic and English auxiliaries and accounts for the distribution of South Slavic clitic clusters. The analysis moves much minor cross-linguistic variation out of the syntax into the lexicon and the level of Phonological Form. Following a critique of various approaches to lexical insertion in Chomskyan models, we adapt Emonds' (1994, 1997) theory of syntactic and phonological lexicalization for a model employing bare phrase structure. We redefine 'extended projection' in this theory, and revise the mechanism of 'Alternative Realization', whereby formal features associated with (possibly null) XP may be realised on another node. Pronominal clitics are one example of Alternative Realization. We claim that the Serbian/Croatian/Bosnian clitic cluster is phonologically lexicalized on the highest head in the extended projection. The clitic auxiliaries in SCB are not auxiliaries, but the altemative realization of features in 1º without categorial specification, hence the distribution of the clitic cluster as a whole. We show how a verb's extended projection may be extended by 'restructuring' verbs, allowing clitic climbing. In Bulgarian/Macedonian, the clausal clitic cluster appears on the highest [+V] head in the extended projection, determined by the categorial specifications of the auxiliaries. In the DP, the possessive dative clitic forms a clitic cluster with the determmer, its distribution determined by the realization of the Dº feature. SCB and Bulgarian clitic clusters require a phonological host in the domain of lexicalization: phonological lexicalization into the Wackemagel Position occurs as a 'last resort'. The treatment of auxiliaries and restructuring verbs m English and South Slavic derives from their lexical entries. Dual lexicalization and bracketing of features in the lexicon allows variation in trace licensing, optional word orders, and minor language-specific phonological idiosyncrasies.