A review is given of the experimental data pertaining to the frequency-dependent (ac) conductivity characteristic of ionically, and electronically, conducting amorphous solids. It is found that, although at first sight the ac behaviour appears to be universal, substantial differences exist between the ac response exhibited by these two classes of non-crystalline material, particularly concerning the temperature dependence of the ac conductivity. The various models that have been proposed to account for this behaviour are summarized, and the probable mechanisms of charge transport responsible for the experimentally observed frequency-dependent conductivity in amorphous solids are emphasized.