This volume represents a modern perspective on the theory of language acquisition. The central concept, which energes from both linguistic theory and acquisition research, is that small pieces of data can be used to set parameters which then produce the variety of human languages found in the world. The articles illustrate a new methodology in the logic of language acquisition. Intricate reasoning from several disciplines is combined: linguistic theory, mathematical learnability, and experimentation. Leading representatives in these fields confront central empirical issues in language acquisition. The major substantive questions addressed are: the acquisition of anaphora. lexical restrictions and the syntax of empty categories. In several papers, the methodology is extended from first to second language acquisition.