Neutrinos are the most elusive elementary particles in our Universe. In this talk, I will first summarize what we already know about neutrinos, and what we would like to know in future. Then, the important role played by neutrinos in astrophysics will be highlighted, and recent progress in the studies of supernova neutrinos and ultrahigh-energy cosmic neutrinos will be reported. As an important cosmic messenger, neutrinos are becoming a very useful tool for us to learn more about the Sun, core-collapse supernovae, and the extremely powerful cosmic accelerators.