  Investigations of the origin of cosmic rays are presented.
  Different methods are discussed: studies of cosmic gamma rays of energy from
30 MeV to about 10^15 eV (since photons point to their places of origin),
studies of the mass composition of cosmic rays (because it reflects source
morphology), and studies of cosmic rays with energy above 10^19 eV
  (for these are the highest energies observed in nature).
