Purpose: This paper aims at a better understanding of expert roles in transdisciplinary projects. Thus, the main purpose is the analysis of the roles of experts in transdisciplinary projects. Design/methodology/approach: The analysis of the ETH-UNS case studies from the point of view of the psychology of expertise and the sociology of professions is based on findings and considerations from the psychology of expertise and the sociology of professions--as both lines of research are concerned with experts and the use of expertise. This paper focuses on projects in the framework of the so-called transdisciplinary case study approach that has been developed at the Swiss Federal Institute of Technology Zurich in the 1990s. Findings: It is claimed that, firstly, system experts provide important information on the local human-environmental system and have to be regarded as serious experts, that is knowledge specialists with a certain responsibility for information. Secondly, decision-making experts run into problems integrating other professionals into transdisciplinary projects and should, therefore, professionalize themselves. Practical implications: The paper encourages the use of residents, etc. as system experts in transdisciplinary projects. Originality/value: The roles of experts in transdisciplinary project are clarified. (Contains 1 figure and 2 tables.)