This study used ERP (event-related potentials) to examine both the role of the L1 and the role of individual differences in the processing of agreement violations. Theories of L2 acquisition differ with regard to whether or not native-like acquisition of L2 features is possible (Schwartz and Sprouse, 1994, 1996; Tsimpli and Mastropavlou, 2007), and the results of previous ERP studies are inconsistent when it comes to whether or not native-like processing is observed in response to L2 agreement violations (e.g., Sabourin, 2003; Tokowicz & MacWhinney, 2005). Furthermore, studies of learners in early stages of L2 acquisition have found variability in the emergence of native-like responses (e.g., McLaughlin et al., 2010; Tanner et al., 2012), but sources of variability have not been investigated. The current study examines responses to gender and number agreement violations in English-speaking learners of Spanish (n=24). Stimuli targeted agreement in three conditions: subject-verb agreement (el barco...flota/*flotan), which is similar in Spanish and English; number agreement on adjectival predicates (la isla...rocosa/*rocosas), a context in which agreement is not instantiated in English; and gender agreement on adjectival predicates (la isla...rocosa/*rocoso), which is unique to Spanish. Grammaticality judgments and ERP responses were also tested for correlations with aptitude scores on the Modern Languages Aptitude Test (MLAT; Carroll and Sapon, 1959) and LLAMA tests (Meara, 2005) and the Raven Advanced Progressive Matrices (Raven, 1965). Results are in line with theories that claim native-like processing is acquirable even for features that are not present in the L1 or are instantiated differently in the L1 and L2. Learners demonstrated similar ERP responses to a control group of native Spanish-speakers (n=12) with regard to all three agreement types, although the response to gender violations was more limited for learners, who also exhibited low proficiency. Additionally, the MLAT and LLAMA were significantly correlated with sensitivity to agreement violations, both in terms of grammaticality judgments and ERP amplitudes, indicating a role for verbal aptitude in L2 processing. No correlations were found for nonverbal aptitude. [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.]