The origin of the ultra high energy cosmic rays (UHECR) with energies above E
> 1017eV, is still unknown. The discovery of their sources will reveal the
engines of the most energetic astrophysical accelerators in the universe. This
is a written version of a series of lectures devoted to UHECR at the 2013
CERN-Latin-American School of High-Energy Physics. We present an introduction
to acceleration mechanisms of charged particles to the highest energies in
astrophysical objects, their propagation from the sources to Earth, and the
experimental techniques for their detection. We also discuss some of the
relevant observational results from Telescope Array and Pierre Auger
Observatory. These experiments deal with particle interactions at energies
orders of magnitude higher than achieved in terrestrial accelerators.Comment: 22 pages, contribution to the 2013 CERN - Latin-American School of
High-Energy Physics, Arequipa, Peru, 6-19 Mar 201