A review is presented on the subject of periodic operation of catalytic reactors by composition forcing. Possible objectives of this mode of reactor operation are increased conversion, improved selectivity, reduced catalyst deactivation and insight into mechanisms of reactor models. Several forcing strategies may be used : manipulating one or more reactant concentrations, or interspersing pulses of inerts between pulses of reactants. These strategies are distinct from the variables in periodic operation, i.e., frequency, wave shape, amplitude, and phase lag. Laboratory-scale equipment for periodic forcing makes use of single reactors along with the control of reactant and/or diluent flows. On an industrial scale, two catalyst beds are used, each operating with different feeds under different conditions. Catalyst transfers between the beds. A large literature has developed over the 25 years since periodic operation was first proposed.