This dissertation investigates the interplay between the implicit knowledge that learners have of a nonnative language and their processing of that language, examining two types of relative clauses (RCs) in English: gapped RCs (e.g., "the man that they hired") and resumptive RCs (e.g., *"the man that they hired him"). Resumptive RCs are considered ungrammatical in English, and yet they are not uncommon in nonnative speech/writing, even from learners whose first language (L1) also disallows them. Phenomena of this type represent a very interesting area of second language (L2) research because they cannot be straightforwardly traced to either L1 influence or target language input, thus raising questions as to whence they emerge in the first place. This project draws on innovative methods from the psycholinguistics literature on adult native speakers to investigate key questions concerning resumptive RCs in adult L1/L2 English: First, is resumption a licit option for relativization in the grammar and/or a means of managing cognitive load during real-time sentence processing? Second, if resumption does aid in RC processing, are facilitation effects observed only in production or also in comprehension? Finally, for L2 learners (L2ers), does performance vary as a function of English proficiency? The participant sample comprises English native speaker controls (ENSs), L1-Korean L2ers of English (KLEs), and L1-Mandarin L2ers of English (MLEs); note that only Mandarin allows resumptive RCs in any of the syntactic environments tested. Participants were distributed across two studies: one on direct object RCs (ORCs), the other on subject RCs (SRCs). Both have three main tasks: an oral elicited production task (EPT) probing processing during RC production, a self-paced reading task (SPRT) probing processing during RC comprehension, and an offline acceptability judgment task (AJT) testing acceptability of the sentence types in the other tasks. The conditions in each task target three increasingly complex RC environments: short-distance, long-distance, "wh"-island. L2ers completed two versions of the AJT--one in English, one in their L1--so that L1 transfer effects, if present, could be identified. All participants also completed a language background questionnaire, an English proficiency C-test, and a short exit survey. L1/L2 results from the EPT and the SPRT provide clear-cut evidence that resumption eases both production and comprehension of ORCs/SRCs under processing strain. For L1 adult processing, this finding contests claims in the literature (e.g., Ferreira & Swets, 2005) that resumption helps only with language production. For L2 research, clear facilitation effects for resumption emerged in both sets of the processing data even when L2ers who consistently accepted resumptive RCs in the English AJT had been removed; this finding that L2 resumption is in many cases a purely processing-based phenomenon rather than a representational one casts doubt on a traditional assumption in the L2 literature (e.g., Hyltenstam, 1984) that is often implicitly held to this day. While a sizable portion of L2ers did consistently accept English resumptive RCs, proficiency effects also arose in the AJT data: Individuals with higher C-test scores were more likely both to reject resumptive RCs--at least in the short- and long-distance environments, where English allows only gapped RCs. These results indicate that with rising English proficiency, it is possible for L2ers to become sensitive to the English prohibition on resumption in RCs. [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.]