Agriculture is facing major challenges related to climate chance, loss of biodiversity and food security. To mitigate these problems, a societal transition towards more sustainable food systems is needed. Agricultural cooperatives play major roles in this sustainability transition. Using a transition theory framework, the chapter presents a narrative review of literature on what cooperatives do, and what they could do, to make agriculture and the whole food system more environmentally sustainable. Besides current activities in marketing farm products and providing inputs - both highly important for supporting members to apply sustainable farming practices - cooperatives can set up dialogues within the membership to develop a shared sustainability vision, engage in networking with external stakeholders to lobby for favorable institutional conditions, and more generally take up the role of sustainability intermediary.