Stomata are an attractive system for modellers for many reasons, and the literature contains a large number of papers describing models that predict stomatal conductance as a function of environmental factors. The approaches and goals of these models vary considerably. This review summarizes these different approaches and discusses their strengths and weaknesses with a focus on mechanistically based models. The critical unresolved questions are highlighted and placed in the context of current research on stomatal physiology. Finally, directions for future research are considered.