Rarefied cosmic plasmas generally do not achieve thermodynamic equilibria, and a natural consequence of this is the presence of a significant population of charged particles with energies well above those of the bulk population. These are exemplified by the galactic cosmic rays, but the importance of these high energy populations extends well beyond that context. I review here some of the basic issues associated with the propagation and acceleration of cosmic rays, especially in the context of collisionless plasma shocks.

• T. W. JONES(Department of Astronomy, University of Minnesota)