This accessible book offers an imaginative journey through the Universe in search of water, founded on the latest science. The author begins by re-introducing the molecule H2O, its physical and chemical characteristics, its cosmic formation and abundance. The book goes on to describe methods by which the presence of water is detected, within the solar system and beyond. The book explores the presence and absence of water in the bodies of the Solar System: in the giant planets, with their rings and satellites, in comets, asteroids and in the terrestrial planets. The author explains how small differences in temperature cause water to exist in different states on different planets - vapour on Venus, liquid on Earth and solid ice on Mars - leading to divergence in the evolutionary paths of the three planets. The book concludes with a look at the important role played by water in studies of habitable exoplanets.