Acquisition of the perfective marker "-le" in Mandarin Chinese is a daunting task for second language learners. To date, there has been no consensus on what it is about "-le" that makes it such a challenge to learners. This study investigates the acquisition of "-le" from both a lexical perspective and a discourse perspective. I test two hypotheses: (1) whether the distribution of "-le" in lexical aspectual class, i.e., state, activity, accomplishment, and achievement, plays a role in the way L2 learners use "-le", as predicted by the Aspect Hypothesis; and (2) whether the distinction in discourse between foregrounding and backgrounding plays a role in the way L2 learners use "-le", as predicted by the Discourse Hypothesis. Three proficiency levels of participants participated in an oral storytelling task: "Frog, Where Are You", in Chinese. The results show that both the Aspect Hypothesis and the Discourse Hypothesis are supported. All of the participants used "-le" to distinguish foreground and background, and they also used "-le" predominately with achievements. Lower-level participants differ from higher-level speakers in that they produced much less "-le" with bounding materials in foreground with achievement predicates. It is concluded that the challenge of acquiring "-le" comes from the syntactic complexity of "-le" with bounding materials, rather than lexical aspect or grounding. [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.]