This book examines key issues in theories of what language is and what happens in the mind during second language acquisition. Chapters provide cutting-edge insights and new empirical findings spanning formal and cognitive linguistics, psycholinguistics and second language development, to understand the role of learner-internal factors in SLA. It is a a companion to Input Matters in SLA (eds. Piske & Young-Scholten, 2008) (9781847691095 pbk, 9781847691101 hbk).