First published in 1987. Three decades of intensive study of language development have led to an enormous accumulation of descriptive data. But there is still no over-arching theory of language development that can make orderly sense of this huge stockpile of observations. Grand structuralist theories such as those of Chomsky, Jakobson, and Piaget have kept researchers asking the right questions, but they seldom allow us to make detailed experimental predictions or to formulate detailed accounts. The papers collected in this volume attempt to address this gap between data and theory by formulating a series of mechanistic accounts of the acquisition of language.