The highest energies in the universe

Ever since their discovery in 1912, cosmic rays have gathered the special attention of the physics community. That attention has grown to its highest levels after the detection of several particles carrying energies in excess of 1020 eV - the most energetic particles ever detected. Today particle physicists and astrophysicists join forces in trying to solve some of the fundamental questions in astrophysics: What is the origin and nature of these particles? Is there an end to their energy spectrum? Which mechanism is able to produce such extraordinary energies? Are these particles relics from the Big Bang epoch? In this paper we present a short tour of this exciting field. (orig.)