The notion of a critical period for acquisition of first and subsequent languages is the topic of this book, which investigates the following questions:

1. What is the evidence for a critical period for language acquisition?

2. Is there a critical period for first language acquisition?

3. Is there a critical period for subsequent language acquisition?

These questions raise corollary issues concerning the nature of language acquisition, variables that drive and constrain it, and the role of biological maturation. The book demonstrates that first language (L1) is in large part susceptible to age constraints, whereas second language (L2) – a term conventionally referring to any language learned after the first – is only indirectly so affected. Evidence from L1 shows a clear effect of age on acquisition, for language is not thoroughly acquired if age of onset passes seven years, and it is acquired with major deficits if age of onset passes twelve. Evidence from L2 acquisition also shows effects of age of onset, but the range of variation due to individual and socio-motivational differences prohibits a strict definition of a sensitive period for L2. Indeed, the L2 competence of expert adult learners, the unequal achievements of child L2 learners, variation of L2 endstate for learners with different L1 and the lack of consistent empirical evidence for a maturational cutoff, all cast doubt on a critical period for second language acquisition (L2A).