The IceCube Collaboration discovered an extraterrestrial neutrino flux of surprisingly large magnitude. I review possible sources for these neutrinos and their constraints, concentrating on extragalactic sources. As alternative, I discuss the possibility that a significant part of the IceCube neutrino flux has a Galactic origin, produced e.g. in decays of PeV dark matter or by a new type of extended cosmic ray source.
AHEP