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 promo-tion by international networks. Drawing on an ethnographic investigation of one of those tribunals-the International Monsanto Tribunal-this article reflects on the many objec-tives they often pursue: strengthening political positions, publicizing environmental and health social struggles, and promoting legal theories. In our case, we show that articulat-ing 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. [opinion tribunal, International Monsanto Tribunal, environment, ecocide, law] In October 2016, the International Monsanto Tribunal took place in The Hague. This event staged and mediatized a trial to assess allegations made against the multinational Monsanto, a firm whose