The grain size, hence the breakdown voltage per eachgrain boundary, could be controlled with additives such as TiO2, Sb2O3, BeO, Al2O3,K2O and SiC. The electrical stability has been improved by adding ZrO2, Cr2O3 or Nb2O5 to bismuth oxide rich compositions. Table 5.1 summarises the effect of various additives on ZnO varistor properties reported in the presently reviewed literature.