The initiatives on sustainable development taken by territorial public authorities are characterized by an emerging institutionalization process, which makes assessment practices necessary. To understand the impact of such practices on sustainable development public policies, this article relies on new institutional theory. Its empirical section focuses on a multi-level study of the discourses of territorial public actors involved in the assessment of sustainable actions. The results show the delicate appropriation of the sustainable development principles by public actors. The assessment becomes a tool to legitimize and structure sustainable actions, defined from a common norm. Renewed modes of governance as well as organizational learning effects thus emerge.