Active galactic nuclei (AGNs) are well known to be some of the most energetic objects in the universe, and are therefore often considered as candidates for the production of extragalactic cosmic rays. Given their extremely high luminosity, AGNs can accelerate cosmic rays to the required energies even if only 1/1000 of their energy is used for this acceleration. There is some observational support for this hypothesis. Analysis of cosmic ray measurements with the Pierre Auger Observatory suggests a correlation between the arrival directions of cosmic rays of the highest energies of more than 5×10 eV and the positions of nearby active galaxies. In 2017, IceCube detected a high energy neutrino with energy 290 TeV whose direction was consistent with a flaring blazar, TXS 0506-056, which strengthened the case for AGNs as a source of extragalactic cosmic rays. Since high-energy neutrinos are assumed to come from the decay of pions produced by the interaction of correspondingly high-energy protons with the Cosmic Microwave Background (CMB) (photo-pion production), or from the photodisintegration of energetic nuclei, and since neutrinos travel essentially unimpeded through the universe, they can be traced back to the source of high-energy cosmic rays.