Loop diagrams in boxes

Methods for computing loop diagrams in confined scalar and gauge theories are developed. We construct propagators with different types of boundary conditions in boxes (rectangular cavities) using the method of image charges, which allows for a separation of the short-distance singularities. The techniques are illustrated by a calculation of the Casimir effect in a box using a covariant gauge, and of the energy shift of a photon in confined scalar QED. The relevance of such a calculation for a proposed model for the QCD vacuum is discussed.