Zirconia ceramics are popularly used for different structural applications from cutting tools to biomedical implants, thanks to their excellent strength and improved toughness with stress-induced transformation toughening. Nevertheless, selecting the right zirconia structural material is often an exercise of compromise. For example, Y-TZP ceramics are strong but less tough than Ce-TZP ceramics and increasing the aging resistance of Y-TZP generally reduces mechanical strength. In this presentation, we will present that grain-boundary engineering by doping tri- or divalent oxides is able to tailor various properties of zirconia ceramics. In our previous work, we have shown that segregating trivalent cations at the zirconia-grain boundaries can effectively improve the aging resistance of Y-TZP ceramics while maintaining their crack propagation resistance and strength. Following the same grain-boundary segregating approach, divalent oxide-doped monolithic Ce-TZP ceramics were developed that combine excellent toughness (>10 MPa·m^1/2), biaxial strength (≥1200 MPa) and damage tolerance with high reliability (m≥30), hereby benefiting from the transformation-induced plasticity. Interestingly, unlike the opaque Ce-TZP-based composite materials, grain-boundary engineered monolithic Ce-TZP allows light transmission, which is important for certain applications like dental restorations.