Both experiment and theory show that the conductivity of a degenerate electron gas in a ‘dirty metal’ tends continuously to zero in the limit oflow temperatures, as the Fermi energy approaches a mobility edge. A derivation is given in terms of the Kubo-Greenwood formula. This is not the case when the metal-insulator transition is induced by a magnetic field, and then a minimum metallic conductivity σmincan be observed. Doped crystalline semiconductors and amorphous and liquid semiconductors are discussed in this context. A brief account is given of two-dimensional systems.