Four specific examples are considered which highlight the role of interfaces in ionic crystals. They also refer to basic differences of solids and liquids which can be traced back to the rigidity and periodicity of the crystalline matter. The examples are : (i) selective blocking of ionic and electronic carriers by specific electrodes, (ii) electrode kinetics at a triple phase (gas, electrode, solid electrolyte) boundary, (iii) conductivity of space charge layers in the direction perpendicular and parallel to the interface and (iv) chemical diffusion through the grain boundary.