This book tackles the question of the symbolic structure of physics, a topic which is implicit in any theory of knowledge, but one that often seems to be ignored as an explicit and central element in today's epistemological debates. The elucidation of this problem should benefit a number of ongoing discussions and can help to avoid much recurring confusion. The contributions to the book have been chosen so as to provide a coherent view, ultimately addressing the relation between the concepts used in science and the real world. The first part provides an introduction to the problem as it arises in physics, and to the modern history of symbols. The subject of the second part is the epistemological discussion taking place between physicists and philosophers about the role of symbols in our knowledge of Nature. The third part addresses key issues related to the methodology of physics and the character of its symbolic structures. The aim of this book is to provide a consistent, if not comprehensive, view, emphasizing aspects that are paradigmatic for the natural sciences as a whole.