Neutrino mass and oscillation

The question of neutrino mass is one of the major riddles in particle physics. Recently, strong evidence that neutrinos have nonzero masses has been found. Though tiny, these masses could be large enough to contribute significantly to the mass density of the universe. The evidence for nonvanishing neutrino masses is based on the apparent observation of neutrino oscillation-the transformation of a neutrino from one type or "flavor" to another. We explain the physics of neutrino oscillation and review and weigh the evidence that it actually occurs in nature, We discuss the constraints on neutrino mass from cosmology and from experiments with negative results. After presenting illustrative neutrino mass spectra suggested by the present data, we consider how experiments in the near and distant future can further illuminate the nature of neutrinos and their masses.