The electrical conductivity is examined throughout the entire range of strongly coupled states of matter: in gases, liquids, and strongly ionized high-density plasmas. These states are classified and the phase diagram is broken up into regions, depending on the nature of the strongly coupled behavior and its occurrence. After a brief discussion of the electrical conductivity of an ideal plasma, the clearest and most thoroughly developed theoretical models are presented. The available experimental data are described and compared with theory. The variations in the electrical conductivity with density and temperature over many orders of magnitude are described.