Intrinsic defects have been produced in yttria- and calcia-stabilized zirconia (YSZ or CaSZ) single crystals either by x irradiation at 300 and 77 K or by thermochemical reduction at 1370, 1670, and 1870 K. The spectroscopic properties of the defects have been studied by electron paramagnetic resonance (EPR), optical absorption, and photoemission techniques. Their thermal stability and bleaching characteristics have also been obtained. In the crystals irradiated at 77 K, an orthorhombic EPR signal with ${\mathit{g}}_{\mathit{x}}$=2.019±0.003, ${\mathit{g}}_{\mathit{y}}$=2.012±0.003, and ${\mathit{g}}_{\mathit{z}}$=2.004±0.002 and axes x?[110], y?[1¯12], z?[11¯1], and an absorption band at λ=465 nm have been assigned to a hole trapped at an oxygen adjacent to a ${\mathrm{Y}}^{3+}$ or ${\mathrm{Ca}}^{2+}$ ion. The trigonal EPR signal with ${\mathit{g}}_{\mathrm{?}}$=1.989 and ${\mathit{g}}_{\mathrm{\perp }}$=1.852 and a band at λ=375 nm, which are produced by irradiation and reduction, are associated with electrons trapped at hexacoordinated ${\mathrm{Zr}}^{3+}$ ions, whereas the absorption band at about 480 nm appearing in the heavily reduced samples is tentatively assigned to electrons trapped at heptacoordinated ${\mathrm{Zr}}^{3+}$ ions. We propose an energy-level diagram for stabilized zirconia that explains the shift of the optical absorption edge, the intrinsic emission, and the bleaching characteristics of the defects.