It has been a challenging experience to restudy this book with a view to issuing it in a second edition. The author has tried to approach the task not merely in terms of mechanics but of fundamental thinking. Did the viewpoint he had previously expressed still seem tenable, or had the development of his own ideas and the widening of his horizons forced any basic modifications? This was the central questions he felt bound to face. He still believes that teaching must be judged by its results, that the specific problem of teaching is of the first importance and that our psycho-logical knowledge can show us how to teach successfully for authentic outcomes. Seven years of working and thinking have confirmed him in these notices. Moreover, the six principles here considered still seem to him satisfactory and practical guide lines, which can be very serviceable in the organization and appraisal of teaching. The changes which have actually been made in this second edition – and they are fairly extensive – can be summarized as clarification, condensation, and modernization. The author's experience has indicated that quite a number of passages in the book were not so clear and understandable as he might have wished, and these have been rewritten. In some case, too, material and discussions that seemed irrelevant or unnecessary had been included, and the author has done his best to eliminate them. As to modernization, an enormous amount of material has appeared, which bears on the general problem of teaching and on the topics treated.