The research presented in this dissertation examined changes in brain activity associated with learning, forgetting and using a second language. The first experiment investigated the changes that occur when novice adult second language learners acquire and forget second language words. Event-related brain potentials were measured while native English speakers learning Finnish performed a lexical decision task on Finnish words, orthographically legal non-words and illegal non-words in three sessions during the first year of instruction and in a follow-up session after instruction had ended. Native Finnish speakers participated in a single control session. The illegal stimuli were formed by violating Finnish vowel harmony which prohibits the co-occurrence of front and back vowels in a word. The results revealed that the learners' brain responses to the vowel harmony violations qualitatively changed during instruction, increasingly approximating the response seen in native speakers of Finnish. Performance in the lexical decision task improved during instruction, but did not reach the level of native speakers. During the post-instruction period, the same stimuli elicited identical, qualitative brain response changes--but in reverse order. In contrast, no significant post-instructional decline in performance was observed. These results show that the neural processes underlying L2 attrition involve a path similar to the one observed during acquisition, but in reverse order, and that L2 grammatical knowledge may be especially vulnerable to attrition. The second experiment focused on qualitative brain response changes shown by native Finnish speakers to vowel harmony violated visual stimuli. When the anomalous stimuli were presented in the context of the participants' native Finnish, their brain responses were sensitive to the grammatical anomaly, but when identical vowel harmony violating letter strings were presented in the context of another language that the participants spoke fluently (English), their brain responses reflected a tendency to respond to these stimuli as simply non-words that were not grammatically anomalous. These findings highlight the dynamic and systematic nature of learning and "unlearning" grammatical knowledge of L2 words, and indicate that the application of such knowledge can depend on the language context. [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.]