Starting with a formalism due to R. A. Craig, we derive a general method for obtaining the interaction energy of a system of charged bodies immersed in an electrolyte. This formalism embodies terms which have previously been obtained from theories of the electrostatics and dispersion forces, unifying and extending previous methods. In the applications considered here, we have specialized to flat plates with small surface charges, obtaining many known results (due to Debye and Hückel, Onsager and Samaras, Verwey and Overbeek). In addition we find corrections to the pressure between overlapping double layers which can be significant compared to the classical expressions.