  High energy (~GeV) positrons are seen within cosmic rays and observation of a
narrow line at 511 keV shows that positrons are annihilating in the galaxy
after slowing down to ~keV energies or less. Our state of knowledge of the
origin of these positrons, of the formation of positronium 'atoms', and of the
circumstances of their annihilation or escape from the galaxy are reviewed and
the question of whether the two phenomena are linked is discussed.
