Catalysis means reducing the activation energy of an elementary reaction step. This requires chemical interaction of the reactant with the catalyst and continuous energy optimisation of both reaction intermediate and catalyst, along the reaction path, in the same way as it is done in quantum chemical calculations. Fluxionality of the atoms and electrons in the vicinity of the catalytic bond therefore supports the reaction. The availability of electrons to form the catalytic bond can be influenced in many ways, e.g. by catalyst particle size, alloying, co-adsorption, support interaction, charging, or application of an electrochemical potential. For optimum activity this bond should be neither too strong nor too weak. Corner and edge atoms or surface defects of the catalyst are often more active than ideal single crystal surface atoms. This means that catalytic bond strength and catalytic activity can be tuned by the particle size and shape, and by specific alloying.