Yielding behaviour and deformation modes are characterized for single-crystal and polycrystalline yttria-stabilized ZrO2 tested in compression from 23 to 800° C. The plastic flow of single-crystal specimens is shown to be orientation and temperature dependent, and is interpreted in terms of dislocation activity, transformation plasticity, and ferroelastic domain switching. Polycrystalline material deforms at room temperature by transformation plasticity, and at intermediate temperatures (∼800° C) by forming unstable shear bands, which flow via grain-boundary sliding and cavitation.