The electrical conductivity of monocrystalline (3.0, 9.5, 12.0 and 17.5 mol% yttria) and polycrystalline (9.9 mol% yttria) yttria-doped zirconia samples was studied using impedance spectroscopy in the temperature range of 250-1200 degrees C. Results show that the bulk ionic conductivity is independent of the P-O2 but varies with the amount of yttrium. The highest conductivities are obtained with monocrystalline zirconia samples doped with 9.5 mol% yttria. This set of results and recent oxygen diffusion data, suggest that different kinds of defects are present in the material, depending on the range of temperature and the amount of yttria. The behaviour of the electrical conductivity of the grain boundaries (with and without alumina) is close to that of the higher doped monocrystalline sample and suggests that associated point defects control grain boundary transport properties. A maximum of conductivity ("composite effect") has been observed when the amount of alumina in the doped (9.9 mol%) polycrystalline zirconia is approximately 2 mol%.