**1. Introduction**

Water is essential to all life on earth, but its managemen<sup>t</sup> is facing increasing challenges due to socio-economic pressures such as population growth and the unsustainable use of water resources. Climate change will further exacerbate the water risk for society and the environment, and water-related extremes such as floods and droughts will increase in the future. The long-term aspects of these future trends and the inherent uncertainty within future projections present water managers with considerable challenges. The technocratic approach of working with fixed design standards for engineering seems insu fficient; the environment is constantly changing, and, as a consequence of this, so too are the boundary conditions on which basis engineered water managemen<sup>t</sup> solutions are developed. Therefore, water managemen<sup>t</sup> is increasingly developing into an adaptive form of decision making, where flexibility, robustness and resilience are key [1]. Moreover, societal processes and the physical water system are increasingly interwoven, and there are little natural water systems not influenced by human activity. These developments require novel approaches in decision sciences, data processing, modelling techniques, catalyzed by the integration of social and natural sciences [2]. The international journal *Water* addresses these challenges as an outlet for cutting-edge inter-disciplinary approaches in water science. In this Special Issue, topics cover broad aspects of water systems, including water science, water quality, management, and governance.
