The MOSA (Metal Oxide Surge Arrestor) comprising ZnO non-linear elements has been taking the place of the conventional gapped arresters on a world-wide scale. The ZnO non-linear element was developed in Japan and the application for the surge arrester was also established in Japan for the first time. Since the first completion of MOSA in 1975, many improvements have been made in the characteristics of the ZnO element, such as life characteristics, nonlinearity, and energy absorption capability. Especially the improvement of life characteristics against continuous ac voltage stress has been significant. In the design of MOSA, thermal stability should be studied carefully in addition to the characteristics of the ZnO element itself. This paper outlines the development of MOSA, the basic characteristics of the ZnO element, and its application as MOSA.