Food systems are in transition to produce more healthy food and become more sustainable. By using interview data from vegetable, dairy, and citrus food systems in Uruguay, this thesis shows how actors in the food system can contribute to transitions. First, producers in organic and conventional vegetable systems organize into different types of producer organizations, each type meets specific producer needs. Second, producer organizations fulfil roles as intermediary that facilitate sustainability transitions, for example by experimenting with sustainable practices, producing organic inputs, or sharing knowledge. Third, producers, processors, and retailers in conventional dairy and citrus value chains contribute to food safety, quality and sustainability transitions by adopting various quality standards and by adjusting their mutual agreements. The thesis provides recommendations for policymakers and practitioners on how actors in transitions may best be supported, for instance by promoting different actors to fulfil distinct but complementary roles in transitions.