Most of Professor Piaget’s work has been written for specialists; much of it is still not translated into English. Too often this has meant that teachers and students learn of the work of the world’s foremost child psychologist at second hand, through the books of his pupils and followers. The two essays which make up this book were written as general entries for an encyclopedia with the interested non-specialist in mind. They cannot fail to be of interest to all who are concerned for the future of education.