The present state of the rigorous field theory design of millimeter wave low insertion loss rectangular waveguide E-plane integrated circuit bandpass filters is reviewed. The theory takes into account the higher order mode interaction between all discontinuities as well as the finite thickness of substrates, fins, and metal inserts. Recent developments in this area include both a generalized design theory and the construction of filters with improved or novel characteristics. The unified algorithm described for the direct combination of multi-port modal scattering matrices requires only one matrix inversion and yields the overall field theory design of complicated filter structures and fiter groups, such as complete millimeter wave multiplexers. High rejection values and expanded second stopband characteristics are achieved by reduced waveguide sidewall dimensions, multiple strips or cavities with decreased cut-off frequency, respectively. Magnetically tunable E-plane integrated circuit filters, where the...