Water played a key role in shaping the Solar System—from the formation of early solids to the processes of planetary and moon formation. The presence of water in molecular clouds influences the initial abundance and distribution of water in the circumsolar disk, which, in turn, affected the water budget of the terrestrial planets and, therefore, their geological activity and habitability. On Earth, surficial and deep-water cycles have largely governed the planet’s geodynamical and geochemical evolu- tion. This issue focuses on the past and present distribution of water within the Solar System and how this important molecule affects astrophysical and geological processes.