Owing to their high internal fields, antiferromagnetic materials have natural resonant frequencies in the millimeter and submillimeter wavelength region of the electromagnetic spectrum. Since these normal modes are circularly polarized and can be tuned by an applied magnetic field, devices similar to the usual ferrite devices are possible at these high frequencies with the application of relatively low fields. The dynamics of a simple antiferromagnetic system are briefly reviewed and the important quantities which characterize antiferromagnetic devices are discussed. The figures of merit for antiferromagnetic resonance isolators and phase shift devices are derived. Experimental data on resonant frequency and linewidth, as well as a typical nonreciprocal resonance absorption trace showing a reverse to forward loss of 10 to 1 in chromic oxide at 140 kMc and at 77°K are presented.