The usage of ZnO arresters in power systems has begun at about 30 years ago. They were created as an evolution of semi conductive ceramics for electronics and became an alternative more efficient in relation to the silicon carbide ceramics used in power surge arresters. Besides the problem related to failures in ZnO arresters, there is also too little information about the natural ageing of ZnO varistors. That’s because the estimated lifetime of a varistor is about 30 years, that’s the approximate age of the first ZnO arresters put into operation in power systems. To complete this overview, it is an usual practice in power companies to dispose off any defective arrester with no further analysis about the failure. So there is not much available information or studies about failure diagnosis in arresters so that these failures could become more easily identified with a thermal inspection. Usually, when defective arresters are submitted to analysis, or when tests of degradation and aging are carried through, there is a series of defects whose occurrence is observed with more frequency. The majority of these defects presents well definite causes and characteristics, so that it is possible that the consequences, in the form of the extreme heating of the arrester, are perceivable through the thermal analysis. In the next topics, the main defects detected in the ZnO arresters will be analyzed.