The catalytic activity and selectivity of the gas-exposed electrode surface of metal electrodes in solid electrolyte cells is altered dramatically and reversibly upon polarizing the metal-solid electrolyte interface. The induced steady-state change in catalytic rate can be up to 9000% higher than the normal (open-circuit) catalytic rate and up to 3 x 10(5) higher than the steady-state rate of ion supply. This new effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) has been already demonstrated for more than 30 catalytic reactions on Pt, Pd, Rh, Ag, Au and Ni surface by using O2-, F-, Na+ and H+ conducting solid electrolytes. There is also a recent demonstration for an aqueous electrolyte system. In this paper the common features of previous NEMCA studies are summarized and the origin of the effect is discussed in light of recent in situ work function and XPS measurements which showed that : (1) solid electrolyte cells with metal electrodes are work function probes and work function controllers, via potential application, for their gas-exposed electrode surfaces; and (2) NEMCA is due to an electrochemically driven and controlled spillover of ions from the solid electrolyte onto the gas-exposed electrode surface. These spillover ions establish an effective electrochemical double layer and act as promoters for catalytic reactions. This interfacing of electrochemistry and catalysis offers several exciting theoretical and technological possibilities.