By combining New Institutional Economics and Social Networks Analysis approaches, this thesis aims at developing a research frame to investigate the role of institutions and networks on the outcomes of agri-environmental policies to control agricultural nonpoint source water pollution. We focus on a specific type of partnership developed in France in the frame of European agri-environmental policy: The Local Agri-Environmental Schemes (LAES). The transaction costs associated with the policy implementation involving multiple public and private actors at different scales are likely to be high. Further, transaction costs incurred within the contracting process have a significant impact on farmers’ decision to enter the agri-environmental contracts. An inter-regional comparative analysis is carried out to test the assumptions made regarding the linkages between institutions, networks structure, network membership, transaction costs and policy outcomes. This also allows for considering the incidence of different institutional features framing the implementation of LAES.