Neutrinos play essential roles in cooling the interiors of massive stars and serve as potential signatures of their evolution towards the eventual core collapse and supernova explosion. The intense neutrino burst from the supernova itself may reveal details of the explosion mechanism and of the formation of a neutron star or black hole. In rare supernovae where high-energy neutrinos are produced, low-energy neutrinos emitted near the central engine may interact with high-energy neutrinos to leave imprints on the latter. I will review the emission and detection of presupernova and supernova neutrinos, and the use of these neutrinos as probes of fundamental physics. I will also discuss how low-energy neutrinos can affect the spectrum and flavor composition of high-energy neutrinos produced in rare supernovae and similar sources.