France is engaged since a few years in a national planning of its actions in favour of Open Science, which reaches a certain maturity in 2022. The first plan was announced in 2018 at the annual LIBER conference. In 2021, a second plan was published. Libraries have taken up the issue from the beginning and their role in Open Science has been decisive. At a local level, institutions regularly integrate library services into their strategies and often entrust them with the preparation and management of strategic issues. At a national level, the Ministry of Higher Education and Research is working closely with libraries on the development of open science and on the determination of (open) public research funding criteria. Supporters of Open Science, first-rate partners and leaders in their field of expertise, libraries have participated in 2018 and 2021 in setting up a national policy that responds to critical issues of openness and sharing of publicly funded research results, then in federating and animating active academic and research communities around these issues. This paper aims to show why research libraries in France are participating in the development of this policy, and how they are contributing to the implementation of actions that ensure its growing success. The French government has based its Open Science strategy on complementary approaches that guarantee a coherent vision : publications, data, codes, open educational resources, evaluation of research, sovereignty issues, taking into account the European and international dimension of Open Science. Libraries have found their place in this ecosystem. Committed to the most concrete actions in the short term, characterized by a reflexive hindsight, endowed with a long term vision, libraries are key players regarding issues related to the dissemination, transmission and preservation of knowledge : they have built their role as strategic advisors to public authorities and ensured their recognition and representativeness within the decision-making bodies. As vectors of innovation and keepers of a structured scientific memory, libraries have shown their capacity to federate and manage academic communities around the challenges of opening up research results, data and publications, showcasing their role as key partners for researchers. This paper shall present national achievements made possible by, with and through libraries: texts, documents, practical guides, examples of researchers, engineers and librarians communities; it shall also give some examples of implementation of these policies within higher education institutions. It will explain through practical cases why and how libraries have played an essential role in the launch of French national policies and are now playing a decisive role in its evolution. It will present the main French policy and strategic directions on Open Science and detail the specific aspects related to the action of research libraries. Il will as a result illustrate how libraries, through their expertise but also thanks to their European and cross-professional vision, have been and are working successfully towards the implementation of Open Science strategies in higher education and research institutions, where the national policy finds support and relays