What are neutrinos? Why does nature need them? What use are they?Neutrinos are perhaps the most enigmatic particles in the universe. Formed in certain radioactive decays, they pass through most matter with ease. These tiny, ghostly particles are formed in millions in the Sun and pass through us constantly. For a long time they were thought to be massless, and passing as they do like ghosts they were not regarded as significant. Now we know they have a very small mass, and there are strong indications that they are very important indeed. It is speculated that a heavy form of neutrino, that is both matter and antimatter, may have shaped the balance of matter and antimatter in the early universe. Here, Frank Close gives an account of the discovery of neutrinos and our growing understanding of their significance, also touching on some speculative ideas concerning the possible uses of neutrinos and their role in the early universe.