Macroscopic QED in linearly responding media and a Lorentz Force apporoach to dispersion forces In this thesis a very general quantization scheme for the macroscopic electromagnetic field in arbitrary linearly responding media is presented. It offers a unified approach to QED in such media. Applying the quantization scheme a theory of the dispersion forces on the basis of the Lorentz force is developed. By regarding the dispersion force as the (ground state or thermal state) expectation value of the Lorentz force that acts on appropriately defined charge and current densities Casimir Casimir Polder and van der Waals forces are united in a very natural way that makes transparent their common physical basis. Application of the theory to planar structures yields generalizations of well known Lifschitz and Casimir type formulas.