Abstract A detailed microstructural examination was conducted on specimens of high purity superplastically-deformed 3Y-TZP (tetragonal ZrO 2 stabilized with ∼3 mol.% Y 2O 3). Several of the microstructural features were similar to conventional superplastic metallic alloys including the retention of an equiaxed grain configuration, evidence for only limited dislocation activity within the grains and the concurrent development of internal cavitation. There was no detectable amorphous phase at any of the grain boundaries or at the triple junctions but there was some segregation of yttria to the boundary regions. It is concluded that there are two types of superplastic Y-TZP materials depending upon whether there is an amorphous phase at the grain boundaries. When an amorphous phase is absent, the behavior is fairly similar to superplastic metals except that an additional mechanism operates at the lower stress levels to impede grain boundary sliding.