The main techniques of studying and designing heterogeneous catalytic processes under forced unsteady-state operation (FUSO) are discussed in this review. Topics included are : a) why FUSOs are theoretically more efficient than conventional steady-state processes ?; b) how can a FUSO be devised ?; c) reverse-flow operation; d) partial oxidation in a circulating fluidized bed; e) processes that could be attractive under FUSO at a commercial scale, but reasons why this has not been successfully accomplished; f) FUSO systems that have not been used on a commercial scale, such as reaction chromatography, input composition cycling, input temperature cycling, and g) what could be done to encourage use of FUSOs by industry ? This review demonstrates the large amounts of data that have been collected on processes proposed for FUSO. Translating these efforts into commercial success becomes possible only when researchers understand thoroughly the fundamentals of the FUSO system they are working on. An overview of past and current research suggests that a catalytic process performed optimally under FUSO will never be less efficient than an optimal steady-state process.