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Given the complex, dynamic, and urgent problems that sustainability science addresses, research approaches are required that not only improve the understanding of sustainability challenges, but also to support action for sustainable development. In this context, transdisciplinary research has established as an approach that aims not only to generate new knowledge, but also to promote the societal relevance and application of research findings through direct collaboration of scientists and societal stakeholders from different fields in integrative research processes. Despite its increasing prevalence in the field, there remains a gap between theoretical ideal-typical models of transdisciplinary research and its actual application within sustainability science. While scholars generally agree that transdisciplinary research is societally effective, there is scattered and partly conflicting evidence on which aspects of transdisciplinary research foster societal impact. Moreover, the extent to which transdisciplinary research contributes to scientific progress is largely unexplored. This thesis aims to contribute to a better understanding of the actual implementation of transdisciplinary research in sustainability science. Following three aims, this work likes to (1.) contribute to the measurability of transdisciplinary research processes as well as their societal and academic outputs and impacts, to (2.) demarcate transdisciplinary research from other modes of research in sustainability science and to (3.) identify and examine the determinants that shape the contribution of transdisciplinary research to societal action for sustainable development and to scientific knowledge production. To serve these aims a mixed methods approach is applied that combines strong quantitative elements with in-depth qualitative analyses that integrate the perspectives of practitioners. This thesis provides a broad set of indicators to describe and assess transdisciplinary research that translate theoretical concepts form transdisciplinarity theory into observable variables. The indicators offer a holistic perspective on transdisciplinary research by representing research mode characteristics, societal as well as scientific outcomes of research projects and their specific context. To theoretically demarcate transdisciplinary research from other forms of research, a narrative literature review first elaborates the differences between ‘normal science’, political use of scientific knowledge and transdisciplinarity in their underlying logics of problem definition, knowledge production and research utilization. Subsequently, these concepts were compared with perspectives and expectations of practitioners in the forest sector on integrative research settings, showing that practitioner perspectives align the most with conceptualizations of political use of scientific knowledge. Moreover, a cluster analysis of data from 59 research projects identified five research modes that empirically demarcate ideal-typical transdisciplinary research from other research modes within sustainability science: (1) purely academic research, (2.) practice consultation, (3.) selective practitioner involvement, (4.) ideal-typical transdisciplinary research and (5.) practice-oriented research. Based on this finding, transdisciplinary research can be characterized as an intensive, but balanced involvement of practitioners. It incorporates not only the needs and goals of the practitioners but also their norms and values. Ideal-typical transdisciplinary research goes beyond mere consultatory research approaches and must be distinguished from what is conceptualized as applied research. Regression analysis of 81 research projects and statistical group comparisons of the five research mode clusters show that societal and academic outputs and impacts vary with specific project characteristics and combinations of project characteristics defined as research modes. The findings indicate that more interactive research modes reach more societal impacts. In particular, the involvement of practitioners in early project phases and the targeted dissemination of the research results positively affect societal impacts. This finding also aligns with practitioner expectations on integrative research and research utilization, provided by qualitative analysis. Moreover, the quantitative results show that scientific outputs and impacts decrease with the intensity of interactions, indicating a trade-off between societal and scientific outcomes and impacts. Overall, the empirical results of this thesis support the claimed effectiveness of transdisciplinary research in providing societally relevant, applicable knowledge and encourage further funding of transdisciplinary research by funding agencies. The relationships discovered in this study between research mode characteristics and societal as well as academic outputs and impacts can help researchers design and reflect on their research and can inform funding agencies in the design of project calls and research programs. However, the observed lower academic outputs and impacts of more integrative research modes raise the question of how to further strengthen the systematic documentation and accessibility of the results of transdisciplinary sustainability research. Additionally, the observed trade-off between societal and academic impacts of transdisciplinary research highlights the need for strategies to mediate between the dual aim of transdisciplinary research to contribute to societal problem solving and scientific knowledge production. Keywords: transdisciplinarity, sustainability science, transdisciplinary research, societal impact, scientific impact, research mode, research evaluation