This thesis is a qualitative and quantitative study of the changes that occurred in the phrase structure and system of pronominal clitics in medieval and renaissance Spanish, with the goal of explaining the basic differences between the syntactic properties of clitics in Old Spanish and their counterparts in the various dialects of modern Spanish. Specifically, I argue that these differences are explainable if we classify OSp clitics as Second Position (2P) clitics, in contrast to their modern counterparts. 2P clitics are treated here as prosodically deficient phrasal constituents that appear displaced from their canonical positions as internal arguments of the verb and are adjoined to a phrasal projection at the left edge of the clause (IP). The elements encompassed under the pre-theoretical notion clitic in modern Spanish, however, are not linked to an argument position via a movement chain; rather, they are proposed to be verbal affixes that enter into inflectional relationships with their coindexed argument positions; alternatively put, they are members of the morphological complex instantiating abstract inflectional features, specifically those of object agreement, and hence inherent components of X0 level type categories. I show that the reanalysis from phrasal categories to X0 related categories took place as a consequence of an unrelated development in the phrase structure of OSp. The gradual loss of a particular mechanism of topicalization, responsible for the manifestation of the patterns associated with the verb-second (V2) constraint in OSp, resulted in a situation where a reanalysis of the parameters of phonological cliticization became inevitable: Lacking a preceding phonological host (namely, the constituent known in the V2 literature as the first position element), 2P clitics went from being inherently enclitic to procliticize by default onto the lexical category immediately to their right, namely, the tensed verbal head. The resulting superficial association between clitic and verb, found in an increasing number of contexts, triggered in turn a reanalysis of clitics in the syntax as elements associated with the verbal inflection, with the manifestation of a series of concomitant effects: loss of interpolation, requirement of strict orders between verbal head and clitic, and clitic-doubling.