Since the early studies [55], the origin of the non-linearity of conductivity has been attributed to the regions situated in the ZnO grain boundaries. This was soon confirmed by thin film experiments [52] and photoconductivity measurements [39]. Several models describing the conduction phenomenon have been developed to give an account of the non-linear electrical properties of zinc oxide varistors (Table 5.2). On the other hand, there are other known mechanisms which have been ruled out due to incompatibility with ZnO properties. The diffusion theory has been excluded [55] since the current in ZnO is strongly voltage dependent. The avalanche breakdown mechanism has also been rejected [27, 39, 53, 55] because it results in a positive temperature coefficient; ZnO is known to exhibit a strong negative temperature coefficient.