Despite being a simple molecule, water has played a key role in shaping the Solar System from the formation of early solids to the processes of planetary and moon formation. Through its astrophysical cycle, water has driven the evolution of protoplanetary disks, which, in turn, has affected the water budget of terrestrial planets and, therefore, their geological activities and habitability. Understanding water's role in diverse natural processes requires expertise in astrophysics, geophysics, and geochemistry. This issue of Elements will introduce the different environments and processes where water is of fundamental importance, as well as its past and present distribution within the Solar System and how this peculiar molecule affects astrophysical and geological processes
