The microstructural development of 3 and 8 mol% yttria-zirconia ceramics during microwave sintering was studied and compared to conventionally heated samples. Microwave heating enhanced the densitication processes occurring during constant-rate heating for both materials. No change was found in the grain size-density relationship for the 8 mol% yttria-zirconia. However, a small, but statistically significant shift favoring densification was found for the 3 mol% yttria-zirconia. Differences in the responses of the two materials are not completely understood but may be due to the differences in the activation energy for grain growth and grain-boundary mobility.