Techniques for the design of magnetically tunable band-stop filters using ferrimagnetic garnet resonators (such as yttrium-iron-garnet spheres) are presented. Design for prescribed response, starting from a low-pass lumped-element prototype filter is outlined. The filter structure consists of a strip line or waveguide with garnet spheres mounted at intervals of approximately a one-quarter-wavelength or three-quarters-wavelength, at the center of the tuning range. Tuning is achieved by varying a biasing magnetic field. Techniques for enhancing the coupling to the garnet-sphere resonators are discussed, and the results of four trial designs which verify the theory are presented. The band-stop filter techniques are shown to also provide a very simple means for measuring the ferrimagnetic resonance linewidth /spl Delta/H of garnet spheres.