Microwave resonance absorption inCr2O3, MnO,MnF2, and CoO above and below the paramagnetic-antiferromagnetic transition temperature is reviewed. The amount of the absorption decreases abruptly in lowering the temperature of the sample through its Curie temperature. The nature of the crystal structure changes associated with the changes in magnetic structure are given for comparison. With the exception of CoO, the alignment of magnetic ions to form the antiferromagnetic configuration results in a deformation along an axis perpendicular to sheets of magnetic ions having parallel spins. Similar results are reported for magnetite.