X-Ray Astronomy : Springer : 9789027702951 : 9027702950 : 31 Dec 1974 : It was about fourteen years ago that some of us became intrigued with the idea of searching the sky for X-ray and gamma-ray sources other than the Sun, the only celestial emitter of high-energy photons known at that time. It was, of course, clear that an effort in this direction would not have been successful unless there occurred, somewhere in space, processes capable of producing high-energy photons much more efficiently than the processes responsible for the radiative emission of the Sun or of ordinary stars. The possible existence of such processes became the subject of much study and discussion. As an important part of this activity, I wish to recall a one-day conference on X-ray astronomy held at the Smithsonian Astrophysical Observatory in 1960. The theoretical predictions did not provide much encouragement. While several 'unusual' celestial objects were pin-pointed as possible, or even likely, sources of X-