The facts of adult second language acquisition (L2A) contrast sharply with those of first language acquisition (L1A). Whereas the attainment of full linguistic competence is the birthright of all normal children, adults vary widely in their ultimate level of attainment, and linguistic competence comparable to that of natives is seldom attested. A reasonable explanation for the facts of L1A and L2A is given by the Critical Period Hypothesis (CPH). In its most succinct and theory-neutral formulation, the CPH states that there is a limited developmental period during which it is possible to acquire a language, be it L1 or L2, to normal, nativelike levels. Once this window of opportunity is passed, however, the ability to learn language declines. Consistent with the CPH are the morphological and syntactic deficits of Genie, who was largely deprived of linguistic input and interaction until age 13 (Curtiss, 1977), as well as the desultory linguistic achievements of most adult L2 learners.