This book gives a broad synthesis of conceptual developments of twentieth-century field theories, from the general theory of relativity to quantum field theory and gauge theory. The author gives a historico-critical exposition of the conceptual foundations of the theories revealing a pattern to the evolution of these conceptions. Theoretical physicists and students of theoretical physics will find in this book an account of the foundational problems of their discipline that will help them understand the internal logic and dynamics of their subject. In addition the book will provide professional historians and philosophers of science, and especially philosophers of physics, with a conceptual basis for further historical, cultural and sociological analysis of the theories discussed. The book also contains much material for philosophical (metaphysical, methodological and semantical) reflection. Finally, the scientifically qualified general reader will find in this book a deeper analysis of contemporary conceptions of the physical world than can be found in popular accounts of the subject.