This dissertation examines Spanish Direct Object clitic pronouns in Argentine spoken Spanish of adults and children (ages 4;0-7;0). It concentrates on (Finite verb + Nonfinite verb) constructions that allow both pre-verbal (Proclisis) and a postverbal (Enclisis) clitic placement. Previous corpus studies have shown that lexical (finite verb), semantic (animacy of the referent), and discourse (topicality) factors constrain variable clitic placement. A corpus study of Argentine Spanish shows that, in agreement with previous research, clitic placement is lexically conditioned in the first place (whereas some finite verbs favor Enclisis, other disfavor it). Additionally, inanimate and low-topicality referents favor Enclisis. This corpus study is used to test the grammaticalization accounts which hypothesize that more grammaticalized and frequent finite verbs should disfavor Enclisis. A construction which meets the two requirements but which still favors Enclisis ("tener que" 'have to') is identified. Drawing on measures of relative frequency and assuming cumulative knowledge of contexts of use, a tentative proposal to account for the distribution of Enclisis in "tener que" is put forward and tested with the most frequent verbs. The results suggest that speakers' knowledge of relative frequencies of use of verbs could inform clitic placement in variable contexts. In the second section, two versions of an Elicited Imitation (EI) task and two Elicited Production (EP) tasks test children's use of clitics in variable structures with three frequent verbs in order to explore if children displayed evidence of the strongest constraints reported for the adults (i.e. lexical and semantic). The results of the EI tasks show that accuracy increases in Proclisis compared to Enclisis. Crucially, finite verb and animacy of the referent have a significant effect on inaccurate imitations consisting of repositioning clitics. The results of the EP tasks also show evidence for finite verb and animacy of the referent impacting participants' responses. Taken all together, the findings are seem to indicate that at least the lexical and semantic constraints which operate on adult speakers are also operative in the children tested. The results are interpreted in terms of item-by-item learning, consistent with Usage-based accounts. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]