AbstractA common approach for estimating fluxes of CO2 and water in canopy models is to couple a model of photosynthesis (An) to a semi‐empirical model of stomatal conductance (gs) such as the widely validated and utilized Ball–Berry (BB) model. This coupling provides an effective way of predicting transpiration at multiple scales. However, the designated value of the slope parameter (m) in the BB model impacts transpiration estimates. There is a lack of consensus regarding how m varies among species or plant functional types (PFTs) or in response to growth conditions. Literature values are highly variable, with inter‐species and intra‐species variations of >100%, and comparisons are made more difficult because of differences in collection techniques. This paper reviews the various methods used to estimate m and highlights how variations in measurement techniques or the data utilized can influence the resultant m. Additionally, this review summarizes the reported responses of m to [CO2] and water stress, collates literature values by PFT and compiles nearly three decades of values into a useful compendium.