Stomata are the major route of gas exchange between the atmosphere and the leaf interior. The size of the stomatal pore is controlled by movements of the stomatal guard cells. The guard cells close the stomatal pore to conserve water during stress. Under more favourable conditions, the stomatal movements optimise CO2 uptake whilst minimising water loss. The movements of stomata are controlled by an extensive network of signalling pathways responding to diverse stimuli. One of the regulators of stomata is the circadian clock. We discuss the physiological mechanisms by which the clock may regulate stomatal movements, and the benefits that circadian regulation of stomatal behaviour may confer to the plant.