With current trends in population migration, international mobility, and connectedness, an understanding of the factors that lead to optimal second language acquisition is increasingly important. Based on Helen Neville’s work, this chapter discusses some of the neurocognitive research on second language processing with a focus on studies utilizing event-related potentials (ERP). The chapter is structured around phonology, semantics, and syntax. For each of these subsystems of language, there is a focus on three factors important for second language processing: age of acquisition (AoA), proficiency, and cross-linguistic influence. We argue for a shift in ERP research from a focus on AoA as a sole factor for describing differences in processing languages to a more comprehensive approach, including proficiency and cross-linguistic influence.