  Astrophysical sources are extremely efficient accelerators. Some sources emit
photons up to multi-TeV energies, a signature of the presence, within them, of
particles with energies much higher than those achievable with the largest
accelerators on Earth. Even more compelling evidence comes from the study of
Cosmic Rays, charged relativistic particles that reach the Earth with
incredibly high energies: at the highest energy end of their spectrum, these
subatomic particles are carrying a macroscopic energy, up to a few Joules.
  Here I will address the best candidate sources and mechanisms as cosmic
particle accelerators. I will mainly focus on Galactic sources such as
Supernova Remnants and Pulsar Wind Nebulae, which being close and bright, are
the best studied among astrophysical accelerators. These sources are held
responsible for most of the energy that is put in relativistic particles in the
Universe, but they are not thought to accelerate particles up to the highest
individual energies, $\approx 10^{20}$ eV. However they allow us to study in
great detail acceleration mechanisms such as shock acceleration (both in the
newtonian and relativistic regime) or magnetic reconnection, the same processes
that are likely to be operating also in more powerful sources.
