This article addresses the industrial application of unsteady-state operation that can be achieved in a gas-solid catalytic reactor by periodically reversing the flow direction (reverse-flow reactor) or by periodically moving the feed position (loop reactor). The combustion of lean VOC-air mixtures and the selective catalytic reduction of NOx are processes that can efficiently exploit the heat and mass storage resulting from these devices. The main questions arising in the design of these reactors are addressed in this article, and general guidelines for selecting the operating variables in industrial-size units are given.