An ancient relic of the Big Bang, neutrinos by the millions fill every cubic meter of space, a ghostly, unseen matrix in which the universe has evolved. Now, new experiments on these elusive particles are changing our understanding of the physical world. The first hint of the true nature of neutrinos was Nobel Prize winner Ray Davis’s surprising discovery that fewer neutrinos come from the sun than were expected from our understanding of how the sun produces its energy. We now know that this is due to “neutrino oscillations,” a macroscopic consequence of the laws of quantum mechanics that govern the sub-atomic realm. Oscillations, in turn, tell us that neutrinos have mass, finally confirming a long-held suspicion. Since Davis’s discovery, we have verified the existence of neutrino oscillations and neutrino mass using neutrinos produced in our atmosphere, in nuclear reactors, and by accelerators.