This chapter focuses on how neurolinguistic evidence, such as electroencephalography/event-related potential (EEG/ERP) and functional magnetic resonance imaging (fMRI) data, can help us adjudicate between various views regarding the critical period debate and how to best account for the ubiquitously noted differences that align with age of acquisition effects in language acquisition/processing. ERPs provide a non-invasive method to investigate electrophysiological correlates of mental processes. The chapter discusses two protocols relevant to brain plasticity and processing: static/structural scans detailing aspects of specific structures and pathways within the brain, and fMRI scans which can document neural processes both at rest and in relation to stimuli. A number of studies, both longitudinal and cross-sectional, have found brain adaptations to language learning in at a variety of stages of acquisition, from hour, to weeks, to months. Most of the evidence for neuroplasticity in adult second language (L2) acquisition stems from longitudinal and training studies.