Stomatal control of transpiration is critical for maintaining important processes, such as plant water status, leaf temperature, as well as permitting sufficient CO2 diffusion into the leaf to maintain photosynthetic rates (A). Stomatal conductance (gs) often closely correlates with A and is thought to control the balance between water loss and carbon gain. It has been suggested that a mesophyll driven signal co-ordinates A and gs responses to maintain this relationship, however the signal has yet to be fully elucidated. Despite this correlation under stable environmental conditions, the responses of both parameters vary spatially and temporally and are dependent on species, environment and plant water status. Most current models neglect these aspects of gas exchange though it is clear they play a vital role in the balance of carbon fixation and water loss. Future efforts should consider the dynamic nature of whole plant gas exchange and how it represents much more than the sum of its individual leaf level components, as well as taking into consideration the long term effect on gas exchange over time.