The symmetry between quarks and leptons suggests that neutrinos should have mass. As embodied in the grand unified theory SO(10) this yields masses that can only be detected by neutrino oscillations. Such oscillations could be very important for supernova physics. Present observations of solar neutrinos when combined with standard solar model calculations imply particular parameters for neutrino masses and mixings. If the solar model is somewhat relaxed quite different possibilities emerge, which yield very different predictions for future experiments.