The highest energy particles found in Nature are cosmic rays. Although
discovered over 90 years ago, their origin remains a mystery. I will
describe why, currently, there is great interest in those particles
with energies close to 1020 eV. The methods of detection will be
explained with particular emphasis on progress that has been made in
construction of the 3000 km2 Pierre Auger Observatory in Argentina.
The present data from several observatories will be summarised and the
status of some controversial results reviewed. The explanations for
what is seen are many. Measurements that appear to eliminate one
involving super-heavy relic particles created in the early Universe
will be discussed.