Over recent years ther has been a marked growth in interest in the study and techniques of cosmic ray physics by astrophysicists and particle physicists. Cosmic radiation is important for the astrophysicist because of the information it can yield about energetic astrophysical processes in the further reaches of the galaxy and beyond. For particle physicists, it provides the opportunity to study neutrinos and very high energy particles of cosmic origin. In addition, cosmic rays constitute the background, but also the calibration source, for searches for exotic hypothesized particles and processes such as monopoles, sparticles and proton decay. Concentrating on the highest energy cosmic rays, this book describes where they may originate, acquire energy, and interact, in large-scale shock waves, in supernova remnants and in accreting neutron stars. It also describes their interactions in the atmosphere and in the earth, how they are studied in surface and very large underground detectors, and what they tell us.