This Ph.D. aims at clarifying the future trajectories of the Walloon dairy sector, based on a study of the strategies and constraints of dairy cooperatives as collective agents. From a systemic perspective, the Ph.D. considers, on the one hand, the dairy cooperatives’ strategies within a given context, and the impact of that context on their strategies. On the other hand, dairy cooperatives are approached as structures of collective action, interplaying in terms of governance with its farmers-members. The focus lies on an in-depth understanding of the factors enabling or disabling certain trajectories, in particular the trajectories of product diversification (productions other than drinking milk, milk powder, butter; productions relying on a definition of milk quality broader than that of industrial standard raw material). The Ph.D. consists of four parts: a historical investigation, an analysis of coordination models in the dairy value chain, a study of the co-existence of cooperative models today, and a zoom on how farmers relate to the dairy value chain. The Ph.D. uncovers that different models of horizontal and vertical coordination may co-exist, beyond the classic model of vertical integration, and that a combination of different models of coordination may support a transition towards a diversification of dairy productions at regional scale. The Ph.D. also highlights a structural tension between the short-term interests of the dairy farmers as milk suppliers and their long-term interests as cooperative members. This tension can influence the strategic choices of cooperatives, which implies that attention must be paid on how dairy farmers relate to collective action. ; (AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 2021