This book takes a hard look at some of the assumptions that are customarily made concerning the role of age in second language acquisition. The evidence and arguments the contributors present run counter to the notion that an early start in second language learning is of itself either absolutely sufficient or necessary for the attainment of native-like mastery of a second language. Another theme of the book is a doubt that there is a particular stage of maturity beyond which language learning is no longer fully possible. In short, the book presents a challenge to those who take it as given that second language learning is inevitably different in its essential nature from language acquisition in the childhood years and that second language knowledge acquired beyond the critical period is in all circumstances and in all respects doomed to fossilize at a non-native-like level.