Magnesia partially stabilized zirconia (MgO-PSZ) is one of the most commonly used engineering ceramics based on zirconia. A detailed discussion about how to identify the various phases in high-resolution transmission electron microscopy images of this material is presented. It shows that in some cases, standard procedures of image simulation are inadequate to interpret these images. By including the effect of astigmatism in both experimental and simulated images, together with the digital Fourier transforms of the images, orthorhombic ZrO2 in [001] orientation was identified. The delta-phase, which has a marked effect on the thermomechanical properties of MgO-PSZ, can most easily be identified by high-resolution imaging in the [130]c zone that coincides with a low-index zone axis of the delta-phase.