As noted in Chapter 1, the low-frequency or 'conventional' circuit theory is ordinarily the best model when the wavelength is large in relation to the dimensions of the system components. With increasing frequency and decreasing wavelength, however, the ability of the circuit model to explain the observed behaviour is no longer adequate for many practical problems. In principle at least, and assuming that the phenomena of interest fall within the scope of electromagnetic theory, these limitations can be overcome by recourse to a different model, namely the Maxwell or field equations. Unfortunately, however, this model is also an extremely complicated one. Here the low frequency concepts of 'voltage' and 'current' are, to a large extent, replaced by the electric and magnetic fields associated therewith and these are then described throughout all space! Although there are applications where this detail is useful, if not essential, there are many others where it is not and the problem is how to modify the model so it represents only the details of interest. The conventional low frequency circuit theory is one such simplification. Moreover, it can be shown that this circuit theory is a consequence of Maxwell's equations under the given conditions.