JEAN PIAGET—best known as developmental psychologist but also philosopher, logician, and educator—is one of the most remarkable figures in contemporary behavioral science. For more than forty years he and his associates have been constructing, in bits and pieces across an enormous bibliography, a broad and highly original theory of intellectual and perceptual development. Like Freudian theory, with which one is tempted to compare it in certain respects, Piaget's theoretical system is a detailed and complicated one, not renderable in a few mathematical or verbal statements. Unlike Freudian theory, however, the system in its totality has not been widely assimilated by others. The major purpose of this book is to present an integrated overview of Piaget's achievements, an overview sufficiently detailed to do justice to the complexity of his theory and the variety of his experimental contributions. This introductory chapter is intended to explain why a book on Piaget is desirable—or at least why it was written—and to summarize the plan or organization which the book will follow. In order to put these matters in context and to set the stage for a detailed description of Piaget's system, it may be useful to examine briefly the man himself—the chronology of his life and achievements.