Modelling of stomatal conductance was a key discussion topic at the workshop held in honour of Ray Leuning's retirement. In this note, we compare two recently published alternative models derived from optimal stomatal theory (Katul et al., 2010; Medlyn et al., 2011). We show that stomatal behaviour clearly departs from that predicted by optimal stomatal theory when Rubisco activity is limiting to photosynthesis. There are two possible alternative interpretations of this observation: either the stomatal behaviour is not optimal, or else the optimisation problem is incorrectly specified. We show that the alternative models effectively follow these alternative interpretations, and consider the implications for our understanding of stomatal behaviour.