The question of why language acquisition is not instantaneous is addressed in terms of two related issues: the logical and the developmental aspects of language acquisition. The role of linguistic theory and research in determining the interplay of these two aspects of grammatical development is examined. It is suggested that the "principles-and-parameters theory" is a useful approach to the explanation of language acquisition in that this approach describes the stages of child language development but limits the number and kind of developmental principles to a set of independently motivated parameters plus lexical/peripheral knowledge. Further research uses of the principles-and-parameters approach for exploring universal grammar, the nature of the developmental process, and second language learning are discussed. (MSE)