This unique book presents a unified, in-depth study of phenomena in photon-matter interactions over a range of many orders of energy. It provides a thorough treatment of relativistic quantum electrodynamics from the quantum field theoretic approach, together with non-relativistic theory in both confined and unconfined geometries. The predictions of QED have been verified to a greater accuracy than any other physical theory. Moreover QED is a paradigm for other gauge theories, and it is presented in such a way that the generalization to other gauge theories is natural. Gauge and Poincare symmetry properties and the non-existence of a photon wave function are thoroughly discussed. Starting from the Dirac equation, the non-relativistic interaction of the electron with the electromagnetic field is derived as an effective Hamiltonian of multipole expansions. Succeeding chapters fully develop relativistic QED and quantum optics. The limitations of QED and the necessity of electroweak theory and quantum chromodynamics are also discussed, and the final chapters are devoted to quantum optics inside cavities, concluding with a study of highly non-classical states of the light field such as photon number, squeezed and two photon entangled states. The book includes worked examples and exercises. In addition to its wide usefulness for theoretical physicists, this volume will help workers in quantum optics to gain a deeper understanding of the foundations of their subject, and field theorists will find concrete examples of open systems, which are beginning to influence their basic approaches.