Completely up-to-date, this is the first comprehensive monograph on metal oxide varistors with a focus on microstructure, conduction mechanisms, device failures, ageing, additive impacts and future varistor systems. As such, it covers the fundamentals and applications of metal oxide varistors, including their macro-characteristics, microstructural properties and the device-internal physical and electrical mechanisms. The author reflects on the achievements made in varistor research and propose new approaches to analyze and predict the macro-characteristics, employing such methods as micro-contact measurements and numerical simulations. In addition, he looks at future directions for varistor research, such as ZnO varistors with a high voltage gradient and low residual voltage and further varistor types based on TiO2 and SnO2.