The articulation between knowledge and actions in three environmental devices: conservation, impact assessment, and restoration. In the actions on nature, the ecological knowledge is decisive and source of legitimisation. The reasons why certain kinds of knowledge are promoted or avoided are nevertheless rarely made explicit. This article aims to clarify the links existing between the historical deployment of actions on nature, the knowledge used, and the conceptions of nature in three key modes of action: conservation planning, environmental impact assessment (EIA), and ecological restoration operations, that all took place in a Mediterranean steppe (La Crau, France). Nature is sometimes conceived as a coevolution between traditional human activities and biodiversity (in conservation), sometimes as a list of species (in the EIA), and sometimes as a functional and reticulated ecosystem (in ecological restoration). Knowledge and action relate to each other depending on the historical context of their institutionalisation as well as on the type of knowledge considered relevant for action according to the conceptions of nature of the involved agents. The mobilised ecological knowledge, yet preeminent in the actions on nature, is guided by contextual factors and conceptions of nature that are not formalised in the discourses.