Opinions tribunals dealing with environmental issues have multiplied over the last several years as a consequence of the rise of international environmental law and its promotion by international networks. Drawing on an ethnographic investigation of one of those tribunals—the International Monsanto Tribunal— this article reflects on the many objectives they often pursue: strengthening political positions, publicizing environmental and health social struggles, and promoting legal theories. In our case, we show that articulating those objectives involved intense work to stage the tribunal's legitimacy. We analyze this work and how it was put to the test during and after the sessions of the tribunal. Our article broadly suggests that environmental opinion tribunals are political arenas where rights and identities are not only asserted but also negotiated and legitimized.