The catalytic activity and selectivity of metal films in contact with electrolytes, both solid state and aqueous, can be varied in situ in a very pronounced and reversible manner via electrical potential application (typically 5 I-2 V) between the catalyst tilm and a counter electrode also in contact with the electrolyte. Catalytic rates can thus be varied by up to a factor of 200. The induced steady-state change in catalytic rate is up to five orders of magnitude larger than the steady-state rate of electrochemically supplied ionic species from the electrolyte onto the catalyst surface. This novel effect of electrochemical promotion or non-Faradaic electrochemical modification of catalytic activity (NEMCA effect) has been studied for over 45 catalytic reactions on Pt, Rh, Pd, Ag, Ni and IrO, catalyst-electrodes using a variety of solid electrolytes (0' , F , Na', H' conductors) and more recently mixed ionic-electronic conductors and aqueous alkaline solutions. In addition to potential technological applications, this new effect allows for a systematic study of the role of promoters in heterogeneous catalysis. In this paper the main features of electrochemical promotion are summarized and the origin of the effect is discussed on the basis of work function measurements and recent XPS, UPS, TPD, cyclic voltammetric, STM and ab initio quantum mechanical investigations.