Problem. In the field of sustainability, scholars and policy-makers herald the transformative power of participation in transdisciplinary (TD) research. The increasing use of TD in science policy is rooted in the expectation that involving practitioners in the research process produces 'socially robust' knowledge and fosters the desired societal change. However, there is a strong discrepancy between these expectations, ideal-typical conceptions of TD, and a limited understanding of the complex interactions that forge participation in knowledge production practices. Aim. This thesis aims to enhance understanding of the complex processes that shape participation practices and their outcomes in TD sustainability research. Methods. This thesis links social theories and literature from sustainability research, Science and Technology Studies, and development research with empirical enquiries into the making of participation. Qualitative and quantitative methods are combined in order to draw a rich picture of researchers' and practitioners' perceptions of participation processes and outcomes. Results. To begin, this thesis offers a novel perspective on participation as a relational space that is dynamically forged through the interplay of structures and processes. Secondly, it identifies structural and actor-related elements that constitute participation spaces, including: (in)coherence with reference systems, resources, timing, expectations, trust, values and power relations. The empirical analyses show that funding bodies, researchers, and practitioners exercise 'power over' actor selection, as well as agenda and rule setting, leading to the exclusion of some knowledges and values. Throughout TD processes, 'power over' intersects with 'power with' and 'power to', highlighting trade-offs between the epistemic and societal goals of participation. Unpacking processes also shows how partly incommensurable values with regard to sustainability shape knowledge production, and how the latter can perpetuate dominant discourses on society-nature relations. Finally, this thesis reveals diverse participation-effect pathways as well as feedback effects. Researchers' and practitioners' diverging perceptions of pathways come with contrasting roles and responsibilities they ascribe to themselves and to others. Conclusions. The results demonstrate that participation processes in TD sustainability research are not only a matter of techniques and methods, but also of political endeavours that must be placed in a broader socio-political context. The value negotiations and power relations that form them, the multiple purposes for which they are mobilised and their role in the governance landscape demonstrate the importance of the politics of participation. Fostering conceptual clarity and empirical knowledge of the dynamics that shape participation in and the effects of TD research, this thesis feeds into a critical sustainability scholarship that is aware of power dynamics and is reflexive towards its own practices. It provides researchers and practitioners with theoretically and empirically-grounded analytical perspectives and guiding questions on the role of structures, actors and power relations in TD practice. These tools allow for unravelling the deep-rooted challenges of participation and support researchers and practitioners in making their assumptions about pathways to societal change visible and negotiable.