How and why do all children learn language? Why do some have difficulties while others are early language learners? What are the consequences of early bilingualism? Is it possible to reach native-like competence in a foreign language? Although we still cannot fully answer these questions, research during the last two decades has begun to solve some pieces of the puzzle. This book proposes an interdisciplinary collection of writings from some of the best specialists across several fields in cognitive science, offering a wide sample of recent advances in the study of first language acquisition, bilingualism, second language acquisition, and disorders of oral language.