Canada Research Chair in Neurocognition of Language，McGill University, Canada，Associate Professor Event-related brain potentials (ERPs) provide an excellent tool to investigate the temporal dynamics of language processing, including the fascinating neural changes that take place when language learners become more proficient in their L2. In my talk, I will present data from a variety of large-scale ERP studies investigating second language acquisition in both artificial languages and natural languages, at different levels of L2 proficiency. I will argue that there is little evidence for a strict ‘critical period’ in the domain of late acquired L2 morpho-syntax and that L2 proficiency rather than age of language acquisition predicts the brain's activation patterns, including "native-like" activity at very high levels of proficiency. The general dynamics of these changes, however, is modulated by factors such as one's first language background (e.g., ‘transfer effects’) and the type of language exposure (e.g., immersion versus classroom instruction). If time allows, I will also present the first ERP data demonstrating how long-term immersion in L2 can affect the real-time processing of one‘s first language.