This book provides a broad overview of parameter-setting theory in first and second language acquisition. It goes on to refine the theory by revisiting and challenging the traditional assumptions that underlie it, based on cross-linguistic language data that cover a range of syntactic and phonological phenomena. From an historical perspective on parameter-setting theory to an introduction to its role in computational linguistics, neurolinguistics, and language change, the reader will find a critique of the most commonly made arguments, as well as an index of all the syntactic, phonological, lexical, and morphological parameters presented in the literature to date. A closer look at the theory itself addresses the following questions: What does a parameter-setting approach to language acquisition entail? What are the underpinnings of the theory? What issues and problems remain to be solved?