The high-temperature deformation behaviour of zirconia single crystals stabilised with yttria is reviewed. Cubic or fully stabilised zirconia (FSZ), which is considered the matrix of high-strength partially stabilised zirconia (PSZ), deforms plastically down to 400°C without confining hydrostatic pressure. The relevant deformation behaviour above about 1200°C is characterised by athermal dislocation motion mainly on cube slip planes and diffusion-controlled recovery. Tetragonal polydomain zirconia or t' zirconia consists of plate-like tetragonal domains alternately stacked to form large colonies. These colonies are arranged in a characteristic way to fill the whole crystal volume. t' zirconia shows ferroelastic behaviour preceding dislocation plasticity. In tension, a tetragonal single crystal forms containing residual defects. The data available indicate that the coercive stress strongly depends on temperature. In situ straining experiments in a high-voltage electron microscope show an instantaneous switching of the individual tetragonal domains with the domain boundaries moving sidewise through the domains. Dislocations moving after the ferroelastic deformation are strongly bowed. The nature of the pinning agents is not clear yet. PSZ crystals are thought to consist of a cubic matrix and precipitates of the tetragonal phase of a structure similar to that of the colonies in t' zirconia. Complete dislocations in the cubic matrix moving on cube planes are partial dislocations in two of the three tetragonal variants of the precipitates. They have then to produce a stacking fault or antiphase boundary like defect. The strong age-hardening and overageing experimentally observed can be explained by a decreasing width of the matrix channels between the precipitates and an increasing width of the domains within the colonies. Recent in situ studies in a high-voltage electron microscope have shown that the precipitates in PSZ may undergo ferroelastic deformation, too. Besides, in a number of cases the matrix of PSZ crystals turned out to be tetragonal rather than cubic. Thus, the formation of a tetragonal single crystal containing residual defects as during the ferroelastic deformation in t' zirconia should affect the subsequent dislocation plasticity. The stacking fault or antiphase boundary like defects have experimentally been observed on 110 planes, however, not on the usual 100 slip planes. Thus, in spite of numerous experimental results a number of questions are still open particularly concerning the plastic deformation of PSZ crystals.