Ultra high energy cosmic rays - An enigma

The methods of detection of high energy cosmic rays are briefly described a nd it is shown that results on the energy spectrum and arrival distribution above 4 x 10(19) eV confound theoretical expectation. Some of the ideas wh ich have been put forward to explain the data are outlined. There is an urg ent need for better statistics as only about 12 events above 10(20) eV have so far been detected. Furthermore the limit to the energy which cosmic ray s can have is not known. In addition to the Pierre Auger Observatory, which is briefly described, the Hi-Res fluorescence detector is about to take da ta and an ambitious satellite observatory (Airwatch/OWL) is under discussio n.