In the past decade, a quantitative theory of the electrical properties of liquid metals has been built up, based upon a model of nearly free electrons scattered by the screened pseudopotentials of the assembly of ions. Within the uncertainties of our knowledge of the pseudopotentials, this simple theory agrees with the experimentally observed resistivity and thermoelectric power of non-transition metals and their alloys. It now seems that higher-order corrections to the n.f.e. formula in such systems are not large enough to be easily observed. Current interest is shifting to systems where the n.f.e. model should not be valid: liquid semiconductors, metallic vapours, metal-ammonia solutions, impurity band semiconductors, and semiconductivity glasses. The experimental situation is not reviewed, but attention is drawn to some basic theoretical questions, such as the nature of the atomic or molecular potentials, the role of electron-electron interaction, the character of the metal-insulator transition, and the quantum mechanical interpretation of such classical physical processes as the localization, percolation, and hopping of electrons in highly disordered materials.