Of particular importance for leaf energy fluxes is that leaf temperature, transpiration, and photosynthesis are linked through stomatal conductance. Current representations of these processes in terrestrial biosphere models recognize that the biophysics of stomatal conductance is understood in relation to the biochemistry of photosynthesis. Stomata act to balance the need for photosynthetic carbon dioxide uptake while limiting water loss during transpiration. Consequently, an accurate depiction of photosynthesis is required to determine the stomatal conductance needed for transpiration and leaf temperature. This chapter develops the physiological theory and mathematical equations to describe stomatal conductance, photosynthesis, transpiration, and their interdependencies. Four main types of models are empirical multiplicative models; semiempirical models that relate stomatal conductance and photosynthesis; water-use efficiency optimization models; and plant hydraulic models.