Abstract. To address the water related problems that society is facing in this time and age, hydrologists can no longer consider the hydrological system as if there is no human influence. The human and water systems are closely intertwined and in this interaction patterns emerge that are the result of two co-evolving systems. The hydrology and the water resources form conditions under which societies develop, but as a result, and in turn, societies influence the functioning of the hydrological system. This calls for a new science that tries to understand these interactions with the objective to explore feasible futures and strategies to guide this co-evolution towards sustainable use of our natural resources. After the closure of the PUB decade on Prediction in Ungauged Basins, the International Association of Hydrological Sciences (IAHS) has taken up the challenge of trying to make fundamental contributions to this new science of integrated hydrological and societal processes under the title of "Panta Rhei", symbolising that everything changes in a perpetual dynamic interaction between people and water. This may be a big step in the science of hydrology, venturing well outside the comfortable boundaries of pristine hydrology, but the urgency of water issues around the globe forces us to do so, and in the process, hydrologists are likely to discover new paradigms, new modelling approaches, enhanced understanding of the human-water system, and hopefully new ways to steer our society towards a more sustainable future.