Water management is an area for several sources of institutional complexity, which have been extensively studied but are still poorly handled in pratice. In this chaper we add to the family of boundary entities a concept of boundary institution, in orderto re-visit the dynamics at stake in participatory modelling. On the basis of a few case studies, we show that participatory modelling, as a process, fits this concept of "boundary institution". A boundary institution is a step above considering the model as a boundary object, because it provides rules for interaction among stakeholders without prior consensus among them. In addition, these institutions provide prototypes to develop other institutions that address complex water management issues or that could help in providing institutional infrastructure (rules, etc. ) to exising boundary organization. Boundary institutions have no tangible infrastructure. Because they are intangible, how boundary organizations actually function will require further research.