Abstract
The paper discusses the methodological questions arising in the study of open quasi-optical devices with resonance structures as selective elements. The spread in characteristic dimensions of the efficient scatterers and the requirements imposed on the frequency band being standard for such electrodynamic objects give no way of solving adequately the analysis and synthesis problems with the help of one isolated method. It is just this domain of theoretical radio physics that involves complete resources of both the frequency-domain and time-domain methods along with the physical results of the resonant wave scattering theory.