The conductivity of solid state conductors is provided by quasifree conduction electrons. Good conductors are substances (mostly metals) with resistivity ρ <10^-6-10^-4 Ohm-cm. For example, aluminium (ρ = 2,5 µΩ-cm), copper (ρ = 1,55 µΩ-cm), and gold (ρ = 2 µΩ-cm) are the most common materials used in electrical engineering, radio engineering and electronics.

Other conductors are electrolytes, where current is carried by ions, and plasma, where the current is carried by free electrons and ions.