Contents Summary 93 I. Introduction 93 II. Influence of the speed of gs responses on A and Wi 93 III. Determinants of the rapidity of gs responses 95 IV. Conclusion 97 Acknowledgements 97 References 97 SummaryStomatal movements control CO2 uptake for photosynthesis and water loss through transpiration, and therefore play a key role in plant productivity and water use efficiency. The predicted doubling of global water usage by 2030 mean that stomatal behaviour is central to current efforts to increase photosynthesis and crop yields, particularly under conditions of reduced water availability. In the field, slow stomatal responses to dynamic environmental conditions add a temporal dimension to gaseous fluxes between the leaf and atmosphere. Here, we review recent work on the rapidity of stomatal responses and present some of the possible anatomical and biochemical mechanisms that influence the rapidity of stomatal movements.