We present results of molecular-dynamics simulations of the thermal conductivity, kappa, of ZrO2 and Y2O3-stabilized ZrO2 (YSZ). For both pure ZrO2 and YSZ with low concentrations of Y2O3, we rind that the high-temperature kappa is typical of a crystalline solid, with the dominant mechanism being phonon-phonon scattering. With increasing Y2O3 concentration, however, the mechanism changes to one more typical of an amorphous system. In particular, phononlike vibrational modes with well-defined wave vectors appear only at very low frequencies. As in amorphous materials, the vast majority of vibrational modes, while delocalized, do not propagate like ordinary phonon modes but transport energy in a diffusive manner. We also rind that the few highest frequency modes are localized and do not contribute to kappa.