How can behavior be explained and predicted? How can it be changed? The theory of reasoned action and the theory of planned behavior provide answers to these questions. This textbook, which is now available in a second, updated edition, provides a concise and understandable introduction to the fundamentals, methodology and empirical findings of the aforementioned social psychological theories. Subsequently, it systematically examines their applicability to questions from communication science and shows how they can be integrated into relevant theories from that discipline. This book is not only suitable for students and teachers of communication science, social psychology and health sciences, but also for all practitioners interested in this subject.