This dissertation examines the acquisitional process of middle se constructions by English- and Korean-speaking L2 learners of Spanish. Although clitic se is well known for its multifunctionality, there is a general tendency in the literature to analyze any use of "se" as a reflexive marker. This tendency is especially prominent in the presentation of se in instructional textbooks. However, this analysis is misleading because it fails to explain the use of se in non-reflexive contexts. From the framework of cognitive linguistics, it is suggested that non-reflexive se is a middle marker whose main function is to focus on the central properties of events, rather than on the subject or object of the sentence. By determining the middle marking function of se, all non-reflexive uses can be generalized into one category, which should not be considered "exceptional cases" for learners to memorize. The explanatory power of the cognitive approach on middle se is supported by some L1acquisition research; however, there is still a lack of L2 studies to determine its applicability. The present dissertation aims to fill this gap in the L2 acquisition of middle se, by comparing two L2 learner groups with different L1 backgrounds, English and Korean, with a group of native Spanish speakers. Since both English and Korean middles are distinct from the Spanish middle system, discrepancies between native and non-native performance were expected. Data are collected via three tasks (a picture book written narration task, an acceptability judgment task, acloze task) designed to examine how the middle marker se is produced and interpreted by three different groups. Results show that native and non-native speakers do not differ significantly in their production of se constructions. It was found that telicity is the main factor which affects se marking. Nonetheless, differences were shown in the interpretation data: the association between middle se and the middleness of the context was stronger for native speakers. Also, the higher the proficiency of the non-native speakers, the more native-like their performance was. Finally, the difference between L1s of the two learner groups did not demonstrate any important significance. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]