This dissertation addresses two questions: (a) how do we explain the limited set of argument structure types?; and b) how do lexical structures relate to syntactic structures? This work is situated within the approach of Hale and Keyser (2002, henceforth H&K), whose purpose is to ascertain the role that structural factors play in the behavior of lexical items.As for the first question, H&K argue that argument structures are restricted by the combination of primitive lexical categories defined solely by structural properties. The present study points out that the restrictive power of H&K's theory is undermined by allowing unrestricted recursive combination of such primitive units. It is shown that by restricting this type of lexical recursion, we can make this problem disappear.As for the second question, this dissertation argues that two adicity-changing processes, transitivization and detransitivization, are key to understanding the relation between lexical and syntactic structures. We investigate these processes in Catalan and Spanish. It is proposed that there are two types of verbal affixes: one plays no role in the argument structure of the verb (but refers instead to its aspectual properties), while the other is responsible for the aforementioned processes. It is also maintained that the affixes of the second type correspond to a functional category that bridges over from lexical to syntactic structures. After identifying these two types of affixes, the present study reanalyzes some data from O'odham, Navajo, Miskitu, Ulwa, and Yaqui, which are problematic to H&K's approach.Finally, this dissertation addresses an important extension of H&K's theory, namely the 'manner index'. Such index is either proximate (bound by the internal argument), or obviative (bound by other than the internal argument.) In H&K's theory, the type of index associated with the root determines a particular structure, which in turn determines the behavior of the corresponding verb. The present study derives two typologies by breaking the determination relation between index and structure. First, by combining the structural type('put'/'get') and the index type (obviative/proximate), we obtain a four-way paradigm. Spanish prepositional verbs present the predicted patterns. Second, we account for two alternations: unergative-unaccusative in Italian, and unergative-transitive in English. Given the explanatory power of the obviative/proximate index, it is concluded that this is a welcome addition.* Available in CD-Rom format: $12.00 (USD)For more GLSA publications or to order on-line, please visit our website: http://server102.hypermart.net/glsa/index.htm.