Since the 2000s, facing the increasing globalization and commodification of common-pool resources, community-based organizations managing water and forests at the local level started to create transnational networks. Their main goal is to get direct representation in global governance arenas and to transform languages of expertise around governance norms and the scales at which they operate. The international involvement of grassroots organizations raises several interrogations: who are grassroots experts and to what extent are they different from technical experts dominating international arenas? At what scale and in what field is grassroots expertise constructed as legitimate? Finally, is transnational grassroots expertise based on a harmonization or a diversification of knowledge and practices?