The present decade opens new frontiers in high energy astrophysics. After the Einstein, Temma, EXOSAT and Ginga X-ray satellites of the 1980s, several satellites are simultaneously in orbit, offering spectacular advances in X-ray imaging at low energies (ROSAT; Yohkoh), as well as at high energies (GRANAT), in spectroscopy with increased bandwidth (ASCA; SAX), and in timing (XTE). While these satellites allow us to study atomic radiation from hot plasmas or energetic electrons, other satellites study nuclear radiation at gamma-ray energies (CGRO), associated with radioactivity or spallation reactions. All these experiments show that the whole universe, from the sun to clusters of galaxies, is emitting X- and gamma rays. This symposium is dedicated to highlighting high energy phenomena, in particular those seen in X- and gamma rays, and is intended to bring together the most recent results, and to discuss and deepen our insight into high energy astrophysics.