Far‐infrared spectroscopy has been used extensively for the study of antiferromagnetic resonance, that is, single‐magnon excitations at the Brillouin zone center. The recent discovery of two‐magnon excitations in antiferromagnets by Halley and Silvera now permits precise measurements of magnon energies at selected points on the Brillouin‐zone boundary. The discovery and theoretical interpretation of the two‐magnon effect is described in this review paper. Experimental results for the antiferromagnets MnF2, FeF2, and CoF2and for the canted‐spin weak ferromagnets NiF2and MnCO3are also presented. The experimental data are discussed in relation to the current theoretical understanding of the two‐magnon effect.