The exponential relationship of rate constant to temperature served as a magnet to attract the attention of researchers in the 1960-1970 to periodic temperature forcing. Early theoretical studies and simulation exercises suggested that significant improvements of rate and selectivity could be expected from this type of unsteady state operation. The first experiments in the late 1970s and early 1980 failed to confirm these expectations, so research on periodic temperature forcing languished. The current interest in microreactors with their greatly reduced thermal capacitance has revived activity in temperature modulation. Recent discoveries of the effect of temperature pulses in trickle-bed reactors have further spurred this activity. This review addresses the question of whether temperature modulation of chemical reactors can significantly affect reactor performance. (C) 2004 Published by Elsevier Ltd.