As the Panta Rhei initiative enters its 4th biennium, we need to remind ourselves that a scientific decade on change in hydrology and society requires the perspectives of those disciplines that have traditionally been concerned with society, the social and policy sciences, as well as society itself. While interdisciplinary conversations have been happening to some extent, transdisciplinary endeavours remain largely undocumented. The type of transdisciplinarity we are interested in here engages, broadly speaking, academic and non-academic perspectives in knowledge production. In this respect, transdisciplinarity is closely related to the traditions of participatory research, public participation and participatory action research. It may actively include stakeholders from different social sectors. Academic and non-academic actors are seen as experts who can collaboratively create new knowledge. These actors may possess different processes of problem solving and communication channels, which are seen as complementary or their conflict as productive. A key challenge is to understand and bring together the different stakeholders to constructively develop innovative and transformative solutions in current policy planning, for instance in drought management, flood risk management, land management or delta management. Under-researched issues in this regard include roles, expectations and motivations of actors in transdisciplinary projects, framing conflicts, power imbalances, actor representation, path dependencies, professional constraints, and inflexibility of research funding and contracting. We encourage contributions with critical reflections of success stories, but we also strongly welcome failed or limited attempts at transdisciplinarity from which we can learn how to advance transdisciplinary science in water and hydrology. Transnational knowledge exchange regarding impacts of different transdisciplinary projects in different contexts will be beneficial in this respect.