This chapter explains the principles that determine electrical properties of matter. Although all the properties of materials—optical, thermal, electrical, magnetic, and mechanical—are related, it is perhaps the electrical properties that most distinguish one material from another. This distinction can be as simple as metal versus nonmetal, or it can involve more exotic properties such as superconductivity. Quantum dots are nanoscale semiconductors that have quite different electrical properties from the corresponding bulk material. The electron-hole pairs are closer together in a quantum dot than in the bulk material, making a situation referred to as quantum confinement. The electrical properties of materials can be different at their surfaces, compared with their interiors. One such example is topological insulators, in which the interior of the material is an insulator, and the surface has conducting states. Hydrogen, which is an insulator at ambient pressure, will become metallic at very high pressures, such as within the core of the planet Jupiter.