Ultra-high temperatures and densities, high-energy particles and intense gravitational and magnetic fields exist in many kinds of objects throughout the universe. During the past two decades, the origin and evolution of these conditions have become central topics of astronomy. In many instances, the conditions are so extreme as to be unattainable in terrestrial experiments; astronomical observations are thus a unique source of empirical information about the physical processes that occur under these circumstances. Much of the information is obtained from observations of x rays, gamma rays, and energetic electrons, nuclei, neutrons, and neutrinos. These, together with gravitational radiation, have come to be considered the province of “high-energy astronomy.”