Water is the common ground between astronomy and planetary science as the presence of water on a planet is universally accepted as essential for its potential habitability. Water assists many biological chemical reactions leading to complexity by acting as an effective solvent. It shapes the geology and climate on rocky planets, and is a major or primary constituent of the solid bodies of the outer solar system. Water ice seems universal in space and is by far the most abundant condensed-phase species in our universe. Water-rich icy layers cover dust particles within the cold regions of the interstellar medium and molecular ices are widespread in the solar system. The poles of terrestrial planets (e.g. Earth, Mars) and most of the outer-solar-system satellites are covered with ice. Smaller solar system bodies, such as comets and Kuiper Belt Objects (KBOs), contain a significant fraction of water ice and trace amounts of organics. Beneath the ice crust of several moons of Jupiter and Saturn liquid water oceans probably exist.