This book is an introduction to the principles and parameters theory of language acquisition. The approach, as developed by Chomsky and his associates, has recently come to play an important role both in empirical work with children and in theoretical explorations of the nature of language acquisition. The introductory chapters contrast the modular principles and parameters theory of language acquisition with older rule-based theories. Atkinson goes on to discuss in detail the application of the theory to questions in acquisition, covering such topics as the syntax of null subjects and the pro drop parameter, the subset principle and its role in parameterized binding theory, experimental investigations of x-bar principles, binding principles, the empty category principle, control theory, theta theory and argument structure, and functional categories and the functional parameterization hypothesis. The book also reviews more general concepts such as the nature of developmental mechanisms, the need to constrain theories of linguistic structure, and the questions of whether language development is a continuous process, and whether early structured speech is symptomatic of a linguistic system.