  Particle acceleration occurs on a range of scales from AU in the heliosphere
to Mpc in clusters of galaxies and to energies ranging from MeV to EeV. A
number of acceleration processes have been proposed, but diffusive shock
acceleration (DSA) is widely invoked as the predominant mechanism. DSA operates
on all these scales and probably to the highest energies. DSA is simple, robust
and predicts a universal spectrum. However there are still many unknowns
regarding particle acceleration. This paper focuses on the particular question
of whether supernova remnants (SNR) can produce the Galactic CR spectrum up to
the knee at a few PeV. The answer depends in large part on the detailed physics
of diffusive shock acceleration.
