Sophisticated software is widely available in the form of mathematical and circuit analysis packages. This computing power can be harnessed to assess the electromagnetic coupling between signals in a system and currents in the structure. To achieve this objective, it is necessary to have reliable circuit models of the coupling parameters. A method of formulating such models is described, illustrating key relationships between circuit theory and electromagnetic theory. It is shown how general-purpose testing can be used to refine any model, and how the results of formal testing can be predicted.