The A(l)O(3) addition to stabilized ZrO2 has been studied for more than 20 years. In this article, literature and new results on the positive and negative effects of Al2O3 additions on the electrical properties of ZrO2 are summarized and analyzed. In particular, a comprehensive grain-boundary conduction model is proposed. The Al2O3 addition always increases the bulk resistivity, mainly because of the formations of defect associates and insulating Al2O3 second-phase particles. The Al2O3 addition within the solubility limit increases the grain-boundary resistivity, as a result of increased grain-boundary space-charge potential; the Al2O3 addition above the solubility limit, however, scavenges the silicon-rich second phase from the grain boundaries, thereby decreasing the grain-boundary resistivity.