The energy density of relativistic particles in our galaxy is comparable to the energy density of the interstellar magnetic field and the turbulent motions of the interstellar gas. This makes cosmic rays, a relativistic gas of charged particles, an important dynamical factor in galactic evolution. The mechanisms for the production of the spectrum of cosmic rays, especially at the highest energies, continue to be explored. Explosive bursts of high-energy photons reveal that the universe can be highly dynamic and offer new windows for exploring the mechanisms of particle acceleration. The observations of flares in the Crab Nebula suggest that magnetic reconnection rather than shocks may be the dominant drivers of such energetic particle bursts. A variety of collective plasma effects accompany the transport of cosmic-rays in the interstellar medium. Also, low energy cosmic rays are important for atomic and nuclear processes in the interstellar gas.
