Expert organisations, such as the Intergovernmental Science-Policy Platform on Biodiversity & Ecosystem Services (IPBES), have become increasingly important in global, regional, and local efforts to manage current environmental challenges. As producers of environmental knowledge assessments, these expert organisations are epistemic authorities in their field of expertise. To achieve and maintain epistemic authority, expert organisations constantly need to reproduce and develop their expertise. By using the first cohort of IPBES’s fellowship program as a case study, the current paper examines the production of expertise and the socialisation of new experts into expert organisations. The paper also examines the importance of these socialisation processes in the institutionalisation of expert organisations. By analyzing interviews, observations, and documents, the current study explores the expected goals, the performance, and the results of the socialisation. The study shows how the fellows learned and acquired new roles and norms. The study also shows that whoever controls the socialisation process also control the production of expertise and the institutionalisation of the expert organisation.