In terms of electric properties, materials are roughly classified into conductors, which can easily transport electric current, and insulators, which can hardly do so. The classification is based on electric conductivity, as shown in Chap. 5. Metals are conductors, and their electric property originates from free electrons that can move freely in the material. On the other hand, electrons in insulators such as mica and glass cannot move because of their bonding to atomic nuclei. Hence, the electric behavior of conductors and insulators is very different. This chapter describes the electric behavior of conductors. Chapter 4 describes that of insulators, which are also called dielectrics or dielectric materials because of their other electric properties.