Since it was introduced to the English-speaking world in 1962, Lev Vygotsky's highly original exploration of human mental development has become recognized as a classic foundational work of cognitive science. Vygotsky analyzes the relationship between words and consciousness, arguing that speech is social in its origins and that only as children develop does it become internalized verbal thought. Now Alex Kozulin has created a new edition of the original MIT Press translation by Eugenia Hanfmann and Gertrude Vakar that restores the work's complete text and adds materials that will help readers better understand Vygotsky's meaning and intentions. Kozulin has also contributed an introductory essay that offers new insight into the author's life, intellectual milieu, and research methods. Lev S. Vygotsky (1896-1934) studied at Moscow University and acquired in his brief lifespan a nearly encyclopedic knowledge of the social sciences, psychology, philosophy, linguistics, literature, and the arts. He began his systematic work in psychology at the age of 28, and within a few years formulated his theory of the development of specifically human higher mental functions. He died of tuberculosis ten years later, and Thought and Language was published posthumously in 1934. Alex Kozulin studied at the Moscow Institute of Medicine and the Moscow Institute of Psychology, where he began his investigation of Vygotsky and the history of Soviet psychology. He emigrated in 1979 and is now Associate Professor of Psychiatry (Psychology) at Boston University. He is the author of Psychology in Utopia: Toward a Social History of Soviet Psychology (MIT Press 1984).