The method that gives more properly the reliability of electric circuits against overvoltage is called the coordination of insulation. For this purpose, it is necessary to achieve the most effective method of insulation among substations, overhead contact system and electric cars. The overhead contact wire to supply electric power to electric cars is constructed 5-8m above the ground. Moreover, electric railways are direct grounding system where rails are laid on the ground. Therefore, they differ from the general three-phase power system. To insulate these electric wires, porcelain insulators have been used conventionally. On the other hand, polymer insulators, which are lightweight and convenient for maintenance, are recently being developed. Furthermore, as one of the technical developments related to the coordination of insulation, optical cables that have excellent properties for electrical insulation are applied to electric railways. This paper describes an outline and the trend of coordination of insulation related to electric railways.