Abstract. Plants growing in environments differing in prevailing humidity exhibit variations in traits associated with regulation of water loss, particularly cuticular and stomatal properties. Expansive growth is also typically reduced by low humidity. Nevertheless, there is little evidence in plants for a specific sensor for humidity, analogous to the blue light or phytochrome photoreceptors. The detailed mechanism of the stomatal response to humidity remains unknown. Available data suggest mediation by fluxes of water vapour, with evaporation rate assuming the role of sensor. This implies that stomata respond to the driving force for diffusional water loss, leaf-air vapour pressure difference. Induction of metabolic stomatal response to humidity may involve signal metabolites, such as abscisic acid, that are present in the transpiration stream. These materials may accumulate in the vicinity of guard cells according to the magnitude and location of cuticular transpiration, both of which could change with humidity. Such a mechanism remains hypothetical, but is suggested to account for feedforward responses in which transpiration decreases with increasing evaporative demand, and for the apparent insensitivity of stomatal aperture in isolated epidermis to epidermal water status. Other responses of plants to humidity may involve similar indirect response mechanisms, in the absence of specific humidity sensors.