Beliefs exert a strong influence on behaviour, decision-making, and identifying and solving problems. Despite their importance, beliefs have until recently not been at the centre of scientific interest. In fact, "belief" is an ill-defined phenomenon. From a transdisciplinary perspective the actual approaches to understanding belief seem incompatible as they attempt to highlight such different topics as "belief - religion", "belief - spirituality", "belief - faith", "belief - knowledge", "belief - attitude", "belief - disbelief", "belief - illusion", and "believing - brain function". This situation contradicts the idea that belief is close to pathological phenomena and that it should be eliminated from scientific discussions. Rather, believing is fundamental for understanding the many problems of every-day life. In fact, the book shows that beliefs are relevant for politics, international affairs, economy, law, or religions also in modern societies. This book presents the increasing scientific interest in beliefs and believing, and reflects the change in focus from the content aspect of belief towards the fluid nature of believing.