Written in the tradition of G. Ludwig’s groundbreaking works, this book aims to clarify and formulate more precisely the fundamental ideas of physical theories. By introducing a basic descriptive language of simple form, in which it is possible to formulate recorded facts, ambiguities of physical theories are avoided as much as possible. In this approach the field of physics that should be described by a theory is determined by basic concepts only, i.e. concepts that can be explained without a theory. In this context the authors introduce a new concept of idealization and review the process of discovering new concepts. They believe that, when the theories are formulated within an axiomatic basis, solutions can be found to many difficult problems such as the interpretation of physical theories, the relations between theories as well as the introduction of physical concepts. The book addresses both physicists and philosophers of science and should encourage the reader to contribute to the understanding of the lasting core of physical knowledge about the real structures of the world.