Recent research on monolingual (L1) and bilingual (2L1) language acquisition is exploring the idea that children's early deviant structures involving omission of obligatory subjects and objects might be due not so much to performance limitations or purely syntactic deficits, but rather to an immature interface between syntax and discourse. For children to become competent speakers of a language, they have to learn the inventory of morphosyntactic constructions available in the language as well as how to use them in pragmatically appropriate ways, i.e. they have to map universal pragmatic principles onto language specific structures and then select only those options that are syntactically possible in the target language. This integration of syntactic information with the appropriate discourse framework causes difficulties for learners in all different situations (L1, 2L1, and L2 acquisition). Taking into account that languages vary in the way discourse notions such as TOPIC and FOCUS are mapped onto syntactic structures, a study of the syntax-discourse interface is also expected to be relevant for L2 acquisition. This study investigates if interface issues can explain deviant structures involving the syntactic and pragmatic realization of arguments in adult French and Spanish L2 acquisition by English speakers, particularly for null and postverbal subjects in Spanish, dislocated and clefted subjects in French, and Clitic Left Dislocations in Spanish and French. More specifically, this study (1) evaluates whether syntax-pragmatics interface phenomena are vulnerable (acquired late and/or difficult to acquire); and (2) tests the claim that the interface between syntax and discourse-pragmatics exhibits more developmental instability than narrow syntax. Findings from the current study support the hypothesis that syntax-pragmatics interface structures are vulnerable. Furthermore, the results revealed that (1) this vulnerability might be due to the interfaces in general and not due to different syntax-discourse mappings in different languages; (2) other factors like syntactic complexity, discourse complexity and rendering in the L1 influence the degree of vulnerability for each structure; (3) no evidence was found that the syntax of a particular construction is acquired before its appropriate use; and (4) improvement was found across certain proficiency levels for syntactic competence, syntax-pragmatics competence and production. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]