Plants show a wide range of responses to changes in atmospheric CO2 concentrations. For example, plants reduce stomatal conductance and increase photosynthesis in response to short term (minutes to hour) increases in atmospheric CO2 and thereby increase water-use efficiency. At intermediate time scales (months to several years), photosynthetic biochemistry and leaf morphology acclimates to CO2, most notably through changes in the number and size of stomata in the cuticle. Atmospheric CO2 changes that persist over long (evolutionary) time scales have been linked to evolutionary changes in leaf gas exchange capacity via coordinated changes in stomatal morphology and leaf vein architecture. This seminar discusses these adaptations from an (evolutionary) optimality perspective and highlights the implications for leaf gas exchange under past, present and future CO2 levels.