Since the publication of Vygotsky's Thought and Language in the United States, a number of North American and European investigators have conducted systematic observations of children's spontaneous private speech, giving substantial support to Vygotsky's major hypotheses -- particularly those regarding the social origins of higher psychological functions. However, there still remain many vital questions about the origins, significance, and functions of private speech: How can social and private speech be validly differentiated? What kinds of social interactions promote the use of private speech? What are the sources of individual differences in the use of private speech? This unique volume addresses these and many other important questions. Characterized by a strong emphasis on original data, it reports on systematic observations of spontaneous private speech in children and adults in both laboratory and naturalistic settings. In addition to its systematic analysis of common methodological problems in the field, the book contains the most comprehensive bibliography of the private speech literature currently available.