This book considers the question of whether, or to what extent, a critical period limits the acquisition of a first language as well as a second language acquired postpubertally. The diversity of opinion on this question is represented in this volume. It is a question that has been approached by researchers working in linguistic theory, evolution theory, language processing, and neurophysiology. The critical period hypothesis for second language acquisition (CPH-L2A) has spawned an abundance of data, ranging from grammaticality judgments to speech samples to event-related brain potentials. These data have lent themselves to interpretation in many ways ranging from theories consistent with theories of access (or lack of access) to universal grammar; as suggesting post-maturational age effects and cross-linguistic (transfer) effects; and as evidence for the tremendous diversity of learner outcomes, ranging from little progress to native-like mastery. Chapters in this volume include a mix of theory, evidence, and cautious argumentation. Contributors are equally divided between the pro-CPH-L2A and anti-CPH-L2A positions. Chapter titles are: (1) "Whys and Why Nots of the CPH-L2A" (David Birdsong); (2) "Functional Neural Subsystems Are Differentially Affected By Delays in Second Language Immersion: ERP and Behavioral Evidence in Bilinguals" (Christine M. Weber-Fox and Helen J. Neville); (3) "Co-Evolution of Language Size and the Critical Period" (James R. Hurford and Simon Kirby); (4) "Critical Periods and (Second) Language Acquisition: Divide et Impera" (Lynn Eubank and Kevin R. Gregg); (5) "Age of Learning and Second Language Speech" (James E. Flege); (6) "Ultimate Attainment in L2 Pronunciation: The Case of Very Advanced Late L2 Learners" (Theo Bongaerts); (7) "Confounded Age: Linguistic and Cognitive Factors in Age Differences for Second Language Acquisition" (Ellen Bialystok and Kenji Hakuta). Extensive references and a subject and author index are included. (KFT)