The microstrip waveguide model has been successfully applied to a host of elementary discontinuity problems generally encountered in microstrip circuits. However, until now the efficiency of the model has never been proved for more complex distributed microstrip structures such as multiple element filters, where both the amplitude and the phase of the computed scattering coefficients for the elementary discontinuity elements can be checked.In this paper it is shown the waveguide model may be used to design microstrip filters, taking into account the frequency dependent parasitic effects of any junction and discontinuity involved. The frequency dependent properties of the junctions and discontinuities considered can also be calculated by the quasi-static technique; as has been shown in many studies, the latter theory is restricted to lower frequencies. The quasi-static approach and the wave calculation obtained also suffer from the effect that radiation effects in the discontinuities have not been taken into account.