One of the central questions in recent second language processing research is whether the types of parsing heuristics and linguistic resources adult L2 learners compute during online processing are qualitatively similar or different from those used by native speakers of the target language. While the current L2 processing literature provides evidence for both qualitative similarities and differences between L1 and adult L2 processing, Clahsen and Felser (2006a, 2006b, 2006c) claimed that the types of syntactic representations adult L2 learners apply during online processing are shallower and hierarchically less detailed, and adult L2 learners rely more on other types of linguistic resources available to them, such as lexical-semantic and pragmatic information. This dissertation aimed to explore these issues to provide more insight into the nature of adult L2 syntactic processing, by investigating how advanced ESL learners varying in their ages of arrival--early learners: ages of arrival between 2 and 9 years old; adult learners: ages of arrival between 18 and 31 years old--deal with relative clause island constructions while processing filler-gap dependencies online in a natural reading environment. In addition, the present study also sought to examine how individual differences in working memory capacity (WMC) influence learners' processing behaviors and use of target language grammars in real time. Twenty-eight advanced adult ESL learners with either Korean or Chinese background and 21 early ESL learners, as well as 24 native English speaker controls participated in an eye-tracking reading experiment and took two different types of automated complex working memory span tests (operation-span & symmetry-span). Results suggested that while the early and adult ESL learners made use of active filler strategies to fill the gap as early as possible in the non-island environment, they rapidly deployed relevant syntactic knowledge of island constraints, thereby avoiding illicit filler-gap formation inside the relative clause islands from early stages of processing, as measured by first-pass reading time, first-pass regression, and regression path duration. Results also suggested that the early ESL learners and native English speaker controls were sensitive to structural cues and gap identifications at the ultimate gap, initiating filler-gap reanalysis processes from early stages of processing, as measured by first fixation duration and first-pass reading time. On the other hand, the adult ESL learners exhibited filler-gap reanalysis effects only during later stages of processing, suggesting that they were not as efficient and immediate as the early ESL learners and native English speaker controls in detecting the need for filler-gap reanalysis. Lastly, individual differences in WMC did not show any significant effect on early and adult ESL learners' processing of island constructions, in that both ESL groups successfully blocked gap postulations in the island environment, by and large, irrespective of their working memory capacities. However, it was found that different WMCs among the adult learners influenced their reading behaviors during filler-gap reanalysis at the ultimate gap, in that adult learners with higher WMC were more sensitive to gap identifications than those with lower WMC, showing more immediate filler-gap reanalysis effect from early stages of processing, as measured by first fixation duration and first-pass regression. These results suggest that early and adult ESL learners' processing of structurally complex filler-gap dependencies in the L2 is not qualitatively different from that of native English speakers. [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 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]