Sensing effects at gas-solid interfaces are discussed and compared with emphasis on solid ionic conductors. Energy schemes or band diagrams for ions and electrons are useful for discussing the thermodynamic and electrical properties of gas-solid interfaces in equilibrium. Absolute electrode potentials are defined and illustrated with results for oxygen electrodes on yttria-stabilized zirconia. Atomistic properties of gas-solid interactions are discussed with regard to the reactions and elementary steps during chemisorption, electrode reaction and electron or charge transfer between solid and adsorbate. Apart from equilibria, nonequilibrium processes can be used for gas sensing. Examples are given for the application of different catalytic properties of electrode materials.