  Galactic cosmic rays are believed to be accelerated at supernova remnants via
diffusive shock acceleration. Though this mechanism gives fairly robust
predictions for the spectrum of particles accelerated at the shock, the
spectrum of the cosmic rays which are eventually injected in the interstellar
medium is more uncertain and depends on the details of the process of particle
escape from the shock. Knowing the spectral shape of these escaping particles
is of crucial importance in order to assess the validity of the supernova
remnant paradigm for cosmic ray origin. Moreover, after escaping from a
supernova remnant, cosmic rays interact with the surrounding ambient gas and
produce gamma rays in the vicinity of the remnant itself. The detection of this
radiation can be used as an indirect proof of the fact that the supernova
remnant was indeed accelerating cosmic rays in the past.
