The catalytic activity and selectivity of metals interfaced with solid electrolytes can be altered dramatically and reversibly via potential application. The increase in catalytic rate can be several orders of magnitude higher than that anticipated from Faraday＇s Law. This new phenomenon of electrochemical promotion is of considerable theoretical and potentially practical importance in heterogeneous catalysis. In this paper the main phenomenological features of electrochemical promotion (or NEMCA effect) are surveyed and the origin of the effect is discussed in view of recent surface spectroscopic and quantum mechanical studies.