This book provides a broad overview of parameter-setting theory in first and second language acquisition and refines parameter-setting theory by revisiting and challenging the traditional assumptions that underlie it, based on crosslinguistic language data covering a range of syntactic and phonological phenomena. From an historical perspective to parameter-setting theory to an introduction to its role in computational linguistics, neuolinguistics, 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 to date in the literature. A closer look at the theory itself adresses 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? The empirical studies carried out to test the null subject parameter and verb movement parameter are reviewed to re-examine longstanding theoretical assumptions as well as the learnability implications for first and second language acquisition.