All the natural electromagnetic radiation reaching the Earth, whether from nearby or remote parts of the universe, brings with it information on the contents or structure of the region in which it originated. This is true of each part of the spectrum, from radio waves through the visible, ultraviolet and x-ray regions, to the realm of high-energy gamma rays which is our subject here. We will be concentrating on celestial gamma rays, with energies above about 10 MeV, which provide clear and in some respects unique information on the high-energy particles and processes in the universe, both currently and in the remote past.