It is a great pleasure to introduce this extraordinary book, which represents a momentous advance in theory and research on private speech. In 1992, Rafael Díaz and I, as coeditors of the most recent edited volume on this topic, noted the inspiration to work on private speech offered by Vygotsky's (1930) seminal observation:

Editors Adam Winsler, Charles Fernyhough, and Ignacio Montero have assembled an eminent cast of contributors whose insightful theorizing, creative methodologies, and provocative new findings lend heightened significance to the melding of speech and activity that – as Vygotsky's insightful remarks point out – enables uniquely human mental capacities to blossom. The contributing authors celebrate and extend a rich contemporary literature of empirical studies on children's spontaneous use of private speech to meet challenges in the intellectual, emotional, and social domains.

We now know much more than we did 15 years ago about the developmental course of private speech; about individual characteristics, adult communication styles, and task conditions that influence its development; and about its contribution to diverse aspects of self-regulation. This book adds immeasurably to the extant knowledge base, providing a veritable treasure trove of new findings, methodological innovations, astute insights, and fruitful new directions.