The Search for Origin of the Highest Energy Particles in Nature

I will outline briefly how cosmic rays were discovered and the work that led to the discovery of the extensive air-showers now used to study cosmic rays as energetic as 10^20 eV. I will explain why there is interest in these very rare particles and illustrate the detection techniques using the Pierre Auger Observatory, a 3000 km^2 instrument, as an example. The latest results on the mass, energy spectrum and anisotropy of the most energetic particles will be described.

No knowledge about cosmic rays will be assumed.