Periodic operation of three phase reactors has been explored for more than two decades. This type of forcing changes selectivity and can increase either conversion or throughput. Experiments and simulation demonstrate periodic flow interruption or variation enhances reaction rates for concurrent trickle beds provided reactants are in the gas phase or can be volatilized under bed operating conditions. Temperature excursions in trickle beds can be controlled by either flow variation or switching the feed between an inert and a reactant. Several approaches to increasing rate through faster mass transfer using flow pulsing have been studied. Pulsing flow can be induced by low amplitude modulation. Periodic switching of flow direction in airlift reactors increases gas hold up and thereby the mass transfer rate. Periodic operation of three phase reactors, thus, appears to be a fertile area for engineering research.