The present paper deals with a "reactivity approach" to complex catalytic reactions. It utilizes data obtained with a reactor and demonstrates the very important role of both rate constants and concentrations of adsorbed species, on catalytic cycle, activity, and selectivity. It develops a link between global kinetics and closed sequence of elementary steps. It emphasizes the aspects of "assisted" catalytic reactions, kinetic "coupling" of catalytic cycles, and selectivity. (C) 2003 Elsevier Science (USA). All rights reserved.