The aim of this introductory paper is to survery current views on the mode of action of promoters in heterogeneous catalysis. Examples of current beliefs on mechanism of promotion have been taken from recent literature and these include support effects, reactant self-promotion, site creation, electronic and geometric effects, activation energies, reactant availability, orders of reaction, synergism, compound formation, bimetallics, particular reaction steps affected by promotion, redox effects and surface attachment of molecules to promoters. Several promising developments in theoretical studies of promotion, the use of Cheveral phases, single-crystal studies, molecular, probes, microengineering and ion implantation point the way towards future developments and understanding.