Similarities and differences between ions and electrons for equilibrium situations relevant in the field of solid state ionics are highlighted. The situations considered are specified according to the degree of interaction, distance from interfaces and sample size. While differences mainly become obvious in kinetic properties such as mobilities, in equilibrium, there is-apart from the density of states and its sensitivity with respect to confinement effects-a far-reaching isomorphism for both types of carriers. While treating electronic carriers in the framework of defect-chemical conception is state-of-the-art. the meaning and usefulness of the 'physical picture' (energy level diagrams) for the description of the ionic carrier effects is explored.