Large-scale environmental assessments involve the participation of many scientific disciplines, even more so when repeated throughout the decades and be implemented in institutionalized context such as the IPCC and the IPBES. Although the purported function of these international organizations is to provide a working interface between science and the decision-making circles at international level, the "boundary work" concerns as much the coordination between scientific disciplines not used to work together and lacking a common language, as it defines the interactions between the world of politics and that of scientists. The development of common standards for scientific evidence and plausibility is crucial, both within and then between the natural and social sciences. The challenge is to produce statements of greater clarity than otherwise found in scientific publications for the use of policymakers, without compromising the established scientific standards. The institutional similarities between these two international organizations are deliberate and originate from the same willingness of the UN member states to retain some control over potentially far-reaching scientific assessments (see chap. 2 above). Institutional differences are largely a consequence of more than two decades of experience between the establishment of the IPCC and that of IPBES, and the creation of a more favourable political "climate" during this time: Today, many member states recognise more clearly the potential for valuable policy support provided by science-policy interfaces as compared to the 1980s. Likewise, lobbying groups from all kinds of political background sense the need to actively relate to the existing science-policy interface. The development of IPBES will hopefully benefit from the IPCC experience in both areas, interaction with policymakers and complex scientific coordination. Therefore, it is important to understand the nature of the IPCC and its limits and shortcomings, and also to understand the differences between the two institutions. The first section of this chapter reviews the social sciences literature devoted to the IPCC and its reports without endorsing all the opinions expressed, to reflect on the kinds of problems and challenges the IPBES will have to face. The second section, based on the personal experience of the second co-author, both in the IPCC and in the nascent IPBES, illustrates these problems and emphasises the specificities of the IPBES in its early stages. The ambiguities of the IPCC "model"< Réduire