This book examines key issues in theories of what language is and what happens in the mind during second language acquisition (SLA), inspiring readers to think in new and exciting ways about language learning and teaching. Chapters, written by both established and rising star scholars, provide cutting-edge insights and new empirical findings on major topics of formal and cognitive linguistics, psycholinguistics and second language development, and offer a coherent, wide-ranging, reader-friendly examination of learner-internal factors in SLA. The first section of the book focuses on issues that are pertinent to our understanding of language acquisition, particularly in relation to syntax. The second section comprises empirical chapters on syntax, the lexicon, phonetics/phonology and language production in English and other languages. These chapters refer to theories and frameworks from within SLA to enable the reader to grasp the key questions and issues that are currently relevant. The final section focuses on research relating to how second language (L2) learners make transitions from one stage of development to the next; it covers state-of-the-art psycholinguistic research concerning how L2 acquisition occurs in real time, and includes discussion of models of L2 development both in and out of the classroom.