Since Noam Chomsky's famous 1959 review of B. F. Skinner's Verbal Behavior, a linguistically based paradigm for research in first language acquisition has been strong, even in some senses, dominant. Even Piaget, whose main concern was the nature of cognitive development in general, did not deny the essential claim of a linguistically based paradigm for the study of language acquisition: ‘I (Piaget) also agree with him (Chomsky) on the fact that this rational origin of language presupposes the existence of a fixed nucleus necessary to the elaboration of all languages’ (Piaget in Piatelli-Palmarini, 1980: 57). This linguistically based paradigm has led to a developed theory of what it is that the child must acquire when s/he acquires language, and to precise scientific hypotheses regarding the nature of this knowledge. These hypotheses can be, and are being, subjected to empirical test, thus advancing the scientific foundations of the field. In this paradigm, the postulation of a ‘Language Faculty’ in the mind of the human species, and in the child, has allowed the formulation of specific components of linguistic knowledge which are now being tested in language acquisition as well as in grammars of languages of the world.