The ionic conductivities, activation energies for mobility, and crystal structures of solid electrolytes are reviewed. The high conductivity of a group of compounds containing simple anions is examined in some detail. For a solid to possess high ionic conductivity there must be: (a) an excess of acceptable lattice sites for the mobile ion, (b) only a small energy difference between the ordered and disordered distribution of the mobile ions over these sites, and (c) a low heat of activation for movement of the mobile ion. These conditions are most readily realized when the cations are monovalent and stable in both 4-coordinated (tetrahedral) and 3-coordinated configurations. Since only Ag I and Cu I satisfy these requirements, their nearly unique conduction properties become understandable. Moreover, these requirements appear to be satisfied only with highly polarizable anions; a characteristic of the anions present in the good conductors.