An empirical model of stomatal conductance is presented. The predictions of conductance correlate with experimental determinations of the responses to several kinds of short-term and long-term perturbations, including irradiance, temperature, CO2 and O2 partial pressures, and leaf age and nitrogen nutrition. The model is not mechanistic and cannot account for the observed functioning of stomata independent of mesophyll. Nevertheless its success in predicting responses in intact leaves means that the mathematical expressions used must be analogous to those describing actual functioning.