Over the course of the twentieth century, nature conservation gradually became both a broadly accepted and highly contentious field of public policy. In particular, national parks (NPs), a major and one of the oldest conservation tools in the European Union, have been contested, and their promoters must justify their existence. We aim to document and analyse how NP managers have sought to make NPs legitimate in the case of France. We first define the notion of legitimacy and present its various sources and types, notably in the case of conservation policies. We then retrace the evolution of the ways legitimacy has been attributed to French NPs. In line with the strong national tradition, legitimacy under the 1960 law on NPs was very much stateand science-based, i.e., essentially substantive. The 2006 reform gave more weight to procedural legitimacy, reflecting the participatory turn of French NPs, which was strongly supported by European guidelines. We finally analyse the tensions between these ways of producing legitimacy and show that the attempts to overcome these tensions have varied from one park to the next. The need to combine sources of legitimacy has generated innovative and contextualized but also fragile ways of creating legitimacy for NPs and implementing European conservation policies.