This volume contains a series of lectures presented at the 5th Course of the International School of Astrophysics held in Erice (Sicily) from July 1st to July 14, 1979 at the "E. Majorana" Centre for Scientific Culture. The course was fully supported by a grant from the NATO Advanced Institute Programme. It was attended by about one hundred participants from ten countries. Since the discovery of the first extra-solar X-ray source in the early 1960's, X-ray astronomy has played an increasingly important role in the study of the Universe, bringing new insight to almost every field of modern astrophysics from stellar evolution to cosmology. Generally speaking, this branch of astronomy is concerned with the discovery, classification and study of "hot matter" in the universe, including high energy non-thermal pheno mena. In particular, X-ray observations appear to provide the main, if not the only, probe to inspect regions where collapsed objects are formed, such as the environment of neutron stars and of black holes in the presence of matter accretion onto the objects themselves. It is significant that the first candidate black hole (Cyg X-I) has been primarily singled out by its X-ray emission. In the same context, it is well known that one of the fundamental problems in modern astrophysics is the understanding of the strong activity taking place in galactic nuclei.

Inhalt
X-Ray Astronomy.- Observations of Supernova Remnants with the Einstein Observatory.- X-Ray Spectra of Supernova Remnants.- Observations of Supernova Remnants in the Large Magellanic Cloud with the Einstein Observatory.- High Resolution X-Ray Spectroscopy from the Einstein Observatory.- X-Rays from the Surface of Neutron Stars.- X-Ray Spectra of Galactic X-Ray Sources.- Study of High Luminosity X-Ray Sources in External Galaxies (M-31).- X-Ray Sources and Stellar Evolution.- Stellar Coronae from Einstein: Observations and Theory.- The Structure and Evolution of X-Ray Clusters of Galaxies.- The X-Ray Spectra of Clusters of Galaxies.- Einstein Observations of the Virgo Cluster.- Cluster Parameters.- Models of X-Ray Emission from Clusters of Galaxies.- X-Ray Structure of the Coma Cluster from Optical Data.- X-Ray Emission from Clusters and the Formation of Galaxies.- Radio Properties of Extragalactic X-Ray Sources.- X-Ray Emission from Active Galaxies.- Einstein Observations of Active Galaxies.- Einstein Observations of Quasars.- X-Ray Spectra of Active Galactic Nuclei.- X-Ray Emission from Galactic Nuclei.- Gas Close to the Radiative Continuum Source(s) in Active Nuclei.- Categories of Extragalactic X-Ray Sources.- The X-Ray Background as a Probe of the Matter Distribution on very Large Scales.- Deep Surveys with the Einstein Observatory.