Recent research in linguistic theory and language acquisition theory has progressed to the point where more detailed hypotheses can be made about the interaction of language learning and linguistic theory. These hypotheses can be used to make more precise predictions about prerequisite knowledge for learning a particular construction and the developmental sequences involved. Two recent proposals in the literature are discussed briefly (Chomsky 1981 and Roeper, in press), with their implications for acquisition research. A study of relative clauses is presented, in which an analysis based on linguistic pretesting allows correlations to be made between the development of relative clauses and the complementizer system which would not be possible if the data were analyzed on the basis of age. (Author)