The importance of water across many disciplines of science and engineering cannot be overstated. Water shapes our blue planet, forms clouds, drives the climate, is a unique solvent in chemistry, the ‘elixir of life’ in biology, a key corrosion agent in engineering, and a complex fluid with a multitude of anomalies in its phase behavior in physics. Despite its importance, a full understanding of the behavior of water in its various forms and systems has remained challenging, due to experimental and theoretical limitations. The goal of the MPGC Water Focus Group is to study water across disciplines – in particular, life sciences, material science, energy research, and earth sciences – with the common goal of understanding, predicting and engineering the behavior and macroscopic properties of complex aqueous systems from a fundamental understanding of molecular-level interactions of water in those disciplines. Thanks to recent technical advances, the combination of simulations and experiments now begin to cover the wide range of length- and time-scales necessary to tackle the complexity of aqueous systems. The fundamental study of water in its many forms in complex environments and circumstances is thus getting within reach, from the molecular to the macroscopic scale. Many of the pressing fundamental research questions regarding water can only be addressed using an interdisciplinary approach. The Focus Group tackles crucial challenges in this important field by combining experimental and simulation techniques and bundling the expertise currently dispersed over different disciplines.