The deformation properties of ZrO2-10mol%Y2O3 single crystals were studied in compression in the <100> direction in the temperature range between 500°C and 1400°C. The flow stress and the strain rate sensitivity of the material were compared with those of ZrO2-10mol%Y2O3, which had been earlier studied in the<112> compression direction by other investigators. The flow stress in <100> orientation exceeds that along <112> at low and intermediate temperatures. The strain rate sensitivities of the two compression directions behave in a similar way, showing a minimum in the temperature range from 800°C to 1000°C. From some of the deformed samples, thin specimens were prepared and investigated in a TEM with an accelerating voltage of 1000 kV. The TEM-observations and the temperature dependence of the strain rate sensitivity of ZrO2-10mol%Y2O3 enabled to determine the dislocation mechanisms, which govern the deformation properties of the material. As the second part of the dissertation, the deformation behavior of ZrO2-15mol%Y2O3 and ZrO2-20mol%Y2O3 was studied in the <112> compression direction, so as to compare the plastic properties of the materials with those of ZrO2-10mol%Y2O3 deformed along <112>. The flow stress of these two materials are higher than that of the material stabilized with 10mol% yttria and is almost independent of temperature. Plastic instabilities were observed for the higher yttria-containing materials, which are explained as the interaction between moving dislocations and atmospheres of point defects.