The IUCN Red List of Threatened Species™ is arguably the most widely recognised tool for assessing species' global conservation status. Given the potential social impact of Red Lists, this research aimed at understanding which kinds of data and expertise flow into the assessments, and what role they play in the process. Informed by theoretical approaches from political ecology and science and technology studies, two recently compiled Red Lists were examined as a case, directly interviewing and surveying the central actors of the Red List process, i.e. scientific experts. By adopting a broad definition of expertise, this study showed that a variety of local expertise (local resource users, resident professionals, and citizen scientists) contributes to Red List assessments, but in a less evident way, and always hierarchically following validation by scientific experts. Resident professionals provided crucial information on all aspects of the Red List; local resource users and citizen scientists played a minor role, except for information regarding plant use and species distribution, respectively. Interviews revealed existing hierarchies of knowledge, in which experts with natural science backgrounds decide over what counts as evidence and whose knowledge counts. Recommendations are made on how local expertise can meaningfully contribute to Red Lists.