This volume is dedicated to Riccardo Giacconi for his sixty-fifth birthday, on the occasion of his being awarded a “Laurea Honoris Causa” in physics at the University of Rome “La Sapienza”. A meeting was held in his honor, and presentations were given that reflected the state-of-the-art of research on subjects related to Giacconi's scientific interests and achievements through the decades. This volume is based upon the papers presented at that meeting. It provides a tribute to Riccardo Giacconi for his outstanding contributions towards opening up the entire field of high energy astrophysics.