The paper presents the results of past and current research of farmer-managed irrigation in two agro-ecological zones of Tras-os-Montes. The final objective of the research is to develop a typology of Farmer-managed irrigation systems (Fmis) which can serve as a tool for improving institutional interventions and designing adequate intervention strategies. The research focused on all aspects of irrigation water management. Both the common features as well as those that differ in the functioning of communal Fmis have been identified. Water allocation principles, water availability and water distribution practices are considered determinant elements in explaining the enormous diversity among communal systems and their environments. This heterogeneity has a direct influence on the rigidity/ flexibility of water use at farm level, which leads to a variety of farmer responses and interventions both within and outside of the communal Fmis. It is concluded that starting from the heterogeneity and dynamics found in Fmis and their environments, institutional interventions might be improved. For this goal intervention strategies have been formulated.