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Ultrahigh energy cosmic rays carry information about their sources and the intervening medium apart from providing a beam of particles for studying certain features of high energy interactions currently inaccessible at man-made accelerators. They can at present be studied only via the extensive air showers (EAS's) they generate while passing through the Earth's atmosphere, since their fluxes are too low for the experiments of limited capability flown in balloons and satellites. The EAS is generated by a series of interactions of the primary cosmic ray and its progeny with the atmospheric nuclei. The exponential nature of the atmosphere spreads the air showers laterally over several hundreds of meters, thus enabling ground-based arrays of relatively inexpensive detectors to record and study them.This book describes the EAS phenomenology, the detectors and techniques used, and the latest results on the energy spectrum and composition of the primaries of EAS's and the results on high energy interactions obtained from EAS studies. It also describes the new TeV and PeV gamma ray astronomy (which has been developing over the past decade) and the newly emerging neutrino astronomy, which are related to the origin of cosmic rays.This book serves as an introduction as well as a reference for researchers in the field.