High energy astrophysics is one of the most exciting areas of contemporary astronomy, covering the most energetic phenomena in the universe. The highly acclaimed first edition of Professor Longair's book immediately established itself as an essential textbook on high energy astrophysics. In this complete revision, the subject matter has expanded to the point where two volumes are desirable. In the first, a thorough treatment is given of the physical processes that govern the behavior of particles in astrophysical environments such as interstellar gas, neutron stars, and black holes. Special emphasis is placed on how observations are made in high energy astrophysics and the limitations imposed on them. The tools of the astronomer and high energy astrophysicist are introduced in the context of specific astronomical problems. The material in Volume 1 leads to a study of all kinds of high energy phenomena in the galaxy and universe given in the second volume. This book assumes that readers have some knowledge of physics and mathematics at the undergraduate level, but no prior knowledge of astronomy is required. The pair of books covers all aspects of modern high energy astrophysics to the point where current research can be understood.