The Intergovernmental Science–Policy Platform on Biodiversity and Ecosystem Services (IPBES) aims to bridge between science and policy by assessing and synthesizing diverse forms of knowledge about biodiversity. With its explicit ambition to include not just natural science, but also social science and humanities as well as indigenous and local knowledge systems, IPBES is operating as a pioneer in uncharted territory. This Review assesses the state of research on IPBES processes in order to identify the challenges and lessons to be drawn from its efforts to include different kinds of experts, stakeholders and knowledge systems. The Review discusses both procedural and substantive dimensions of inclusiveness and identifies a paradox between on the one hand IPBES’ demand for diversity and on the other hand its aim of achieving consensus. Specifically, it illustrates how IPBES’ orientation towards consensus in decision-making and in assessment work shapes and constrains diversity and inclusiveness. This finding has implications for environmental assessment processes within and beyond IPBES and suggests a need to open up procedures and practices of participation and inclusion in order to accommodate pluralism, contestation and incommensurable perspectives and knowledge systems.