Cosmic rays are highly relativistic particles that constantly rain down on us from outer space. The energy spectrum of cosmic rays extends over more than 10 decades of energy, following an almost perfect power law. The fluxes are quite large at low energies (about 1 particle/m²/s at 1 GeV) but extremely small at the highest energies (1 particle/ km²/century around 10²⁰ eV). When high energy cosmic rays hit the atmosphere, they interact producing a shower of billions of particles which can cover an area as large as 10 km². Cosmic ray physics has an already century long history! Motivated by the information these particles can bring us about the Universe, cosmic ray physicists study deeply all the features of this nearly featureless spectrum (the knee, the ankle, ...). In experiments dedicated to the different energy ranges, they try to understand the origin (galactic, extragalactic, which sources?) and nature (proton, iron, photon, ...?) of these particles. Here we are interested in the far end of the spectrum: energies of the order of 10¹⁹ - 10²⁰ eV, far above what is reachable in present and planned human made accelerators! These highest energy cosmic rays are a mystery and a challenge. There is no scientific consensus on how and where such energetic particles originate. In particular, for energies close to 10²⁰ eV the interaction of cosmic rays with the cosmic microwave background should quickly degrade their energy. They should thus come from relatively close and in a straight line, practically not deflected by the galactic and intergalactic magnetic fields... But where are the sources? Up to now only a few tens of events have been collected in Volcano Ranch (the pioneer experiment), AGASA, Fly's Eye, HiRes. The features of the spectrum in this energy region are poorly known, and no clear sources have been identified. Their extreme energy and mysterious origin makes very high energy cosmic rays a window to the Universe, but also an opportunity for particle physics. The first interactions in the atmosphere take place at energies in the centre-of-mass up to 450 TeV, to be compared with the 14 TeV of the LHC! Of course the luminosity and detector capabilities are poor compared to accelerator experiments, but the window to high energies is truly unique. Extensive air showers can be detected on ground by an array of charged particle detectors. They can also be measured by detecting the fluorescence light track produced by the excitation of the air nitrogen molecules along the shower track. Given the very small fluxes, building detectors represents a great challenge and requires enormous detection areas. With a detection area of 3000 Km² and hybrid detection capabilities, the Pierre Auger Observatory represents an immense step forward in this domain!