the so-called 'ice frontier' determining the natures of the terrestrial and giant planets. The book explores the presence of water in the various bodies of the Solar System: in the giant planets, with their rings and systems of satellites, in comets, asteroids and in the terrestrial planets. By tracing the history of water in the atmospheres of Mars, Venus and the Earth, the author explains how small differences in temperatures, causing water to exist in different states on different planets - vapour on Venus, liquid on Earth and solid ice on Mars - have led to a great divergence in the evolutions of the three planets. The story of water on Mars, an aspect of great topical interest, offers an insight into the possibility (still only a theory) that there was once life on that planet. The book concludes by looking at the important role played by water in studies of habitable exoplanets.