The successful microwave sintering of zirconia demonstrates the necessity to understand both the materials and electromagnetic field aspects of microwave processing. It was difficult to produce crack-free parts in the multimode microwave furnace employed in this investigation. Nonuniformities in the microwave field, and dielectric properties that increased rapidly with temperature, produced “hot spot” in the parts, which led to differential sintering and subsequent cracking. To produce crack-free sintered parts, an indirect heating method was developed that eliminated the severe differential heating. Using this indirect heating method, it was demonstrated that the sintering temperature of zirconia could be lowered from 1375° to 1200°C by microwave processing and that the resulting grain size was finer.