Nature-based Solutions (NbS) is a concept that is being used internationally and nationally to reframe policy debates on biodiversity conservation, climate change adaptation/mitigation strategies, along with the sustainable use of natural resources such as water. The International Union for Conservation of Nature (IUCN) defines Nature-based Solutions as "actions to protect, sustainably manage and restore natural or modified ecosystems that address societal challenges effectively and adaptively, simultaneously providing human well-being and biodiversity benefits". NbS builds on and supports other closely related concepts such as the ecosystem approach, ecosystem services, ecosystem-based adaptation/mitigation, and green infrastructure. In the context of flood risk management, we observe variable definitions of Nature-based Solutions used by public agencies, who may use this new concept interchangeably with others existing concepts. Our research project aims at analyzing and comparing how different countries, such as France and the US, define and implement these projects qualified as “Nature-based Solutions” in practice looking at: (1) how France and the US are developing the concept of NbS, in particular to see how NbS is defined by different agencies and institutions in each country, (2) how NbS projects are implemented in terms of governance and public participation, (3) what metrics are used to assess and evaluate the social and environmental impacts of these projects, and (4) how inhabitants perceive NbS projects. This presentation will highlight some results of this three years research project (2021-2023). More specifically, we will aim at: i) highlighting how NbS concept is being institutionalized at the national scale between the US and France; ii) examining how NbS are being implemented in practice at local scales, in terms of governance (e.g. cross-sectoral collaboration capacity of NbS) and public participation. At the national level, the first results show that the concept of NbS is being implemented in both countries through varying definitions. In the US, the concept has been actively promoted by the NGO The Nature Conservancy and is now adopted by many different federal agencies (NOOA, USACE, EPA or FEMA). Although the government acknowledges other co-benefits such as water quality, reducing heat, or sustaining biodiversity, this new concept seems to have a strong focus on mitigation strategies, especially regarding to flood risk, and as an alternative solution to aging gray infrastructure. There is no common definition at a federal level and some agencies use this new concept interchangeably with other existing concepts. In France, NbS implementation relies more on biodiversity conservation and climate adaptation policy sectors, and is referring to IUCN definition as a common ground. However, there is little appropriation of the concept from the flood policy sector. NbS are being implemented through incentive mechanisms (information, coordination, financial reporting, benchmarking…) at national or regional levels, rather than command-and-control mechanisms. Through highlights from case studies in France and the US (urban and non-urban), we will discuss whether NbS are enabling inclusive cross-sectoral governance and open public participation in comparison to traditional technical infrastructure.