From a public health perspective, and with a long-standing background in interactive knowledge-to-action research, I will first focus on the challenges of structural change and potential means of collective action to address them. I will introduce a model of the interplay between structure and agency as a theoretical frame of reference. I will then use a case study in health promotion to demonstrate how a transdisciplinary approach to co-production ("cooperative planning") can serve as a mechanism for structural change. The second part of the presentation deals with certain challenges of transdisciplinary research. In particular, the scientific impact of such research will be examined. As a first result of an international fellow group of tdAcademy on this issue, we propose to distinguish between an epistemic, an ethical and an organizational dimension of scientific impact. Focusing again on my case study, concrete examples of all three dimensions will be presented. I conclude with some preliminary thoughts on a possible fourth ("strategic") dimension.