High-hardness structural materials such as fine ceramics and cemented carbides are widely used as materials in industrial tools, electronic components and precision instruments. However, these materials are difficult to machine through conventional machining processes because of their high hardness and brittleness. Electrical discharge machining (EDM) is used for the precision machining of hard materials. Recently, it has also be used for machining insulating ceramics by the assisting electrode method. In the EDM of cemented carbides and conductive ceramics, the machining rate affects their mechanical properties and chemical reactions with discharge phenomena. When insulating ceramics are machined by EDM, their surface becomes covered with an electrical conductive layer formed by cracked carbon. The conformation of this layer differs according to the ceramics type, and affects the machining characteristics. In this paper, we describe the EDM characteristics of high-hardness structural materials such as electrical conductive ceramics, cemented carbide and insulating ceramics.