Why are most public policies so hard to reform? How comes some are so drastically changed at some point? This article proposes a model aimed at explaining public policy change with a specific focus on ideas, discourses and controversies as explanatory variables. Specific analytical tools are proposed, among which are included the notions of arenas and forums as core elements to grasp the variety of controversies at stake. The model helps us to understand the dynamics of production, progressive selection and institutionalization of ideas -i.e. their transformation into public policy instruments, as well as their feed-back effects. These analytical tools are first theoretically defined and described, and the model is then placed in the framework of a multi-governance political system (national, regional - European Union, international levels) and a range of institutional contexts (sectoral, non-sectoral). The proposed model is finally applied to the case of the European common agricultural policy and its reforms.