Purpose – This paper aims at presenting the theoretical concepts of the transdisciplinary case study approach (TCS), which is a research and teaching approach developed and elaborated at the Swiss Federal Institute of Technology (ETH), as a means of transition support. Design/methodology/approach – The paper reveals the historical roots of case studies, transdisciplinarity and sustainable development as teaching and research paradigms. The TCS approach is presented, which has been developed at ETH for supporting transition management of regional, urban, and organizational systems. This approach is entrenched by an ontology that reveals the basic characteristics of ill-defined transition problems, an epistemology that refers to Probabilistic Functionalism and distinguishes between multi-layered systemic and normative epistemics, a methodology that includes a set of methods for case representation (including modelling and projection), assessment, and strategy building, and a project management model that refers to more than a dozen TCSs in the field of sustainable development. Problems of validity of TCSs as a research methodology are discussed. Findings – Three major strengths of the TCS approach presented in the paper are: that it is based on three sound paradigms, which focus on different, relevant characteristics of complex, human-environment systems; i.e. the case study approach, transdisciplinarity and sustainable development, that it is strictly organized according to an elaborated and consistent theoretical framework that includes ontological, epistemological, methodological, and organizational considerations, and that it is itself subject to an ongoing inquiry and adaptation process. All theoretical considerations of the paper are clarified be elaborated examples from the more than 10 years experience with TCS of the authors. Practical implications – The paper gives a comprehensive overview of the theoretical foundation of TCS that might assist other scientists engaged in case study research and teaching to further develop their approaches. Additionally, relevant topics for further research in the field of TCS are presented which hopefully induce an inspiring discussion among case study researchers. Originality/value – As far we know, this paper is one of the first that presents a comprehensive and theoretically sound overview of applying transdisciplinary case studies as means of sustainability learning. Thus, it can be seen as a first, crucial step for establishing the new research field of TCS research and a sound research community of complex, transdisciplinary problem solving towards sustainability learning.