Gamma-ray astronomy comprises the view of the Universe through what is essentially the last of the electromagnetic windows to be opened. All other windows from radio right through to X-rays have already been opened wide, and as is well known their respective astronomies are quite well developed – and the views there are very rich. Gamma-ray astronomy promises to be likewise; the strong link of γ-rays to very energetic processes and the considerable penetration of the γ-rays see to that.

Admittedly one deals with a small number of photons in this new window and yet a considerable amount of progress has already been made; hopefully this progress will shine through in what follows.

It is usually necessary to make a selection of topics when writing a book, and the present one is no exception. The selection made here reflects both the interests of the authors (both of whom are cosmic ray physicists) and the perceived needs of the subject. The authors' interests and, no doubt, biases show through in the areas in which they have themselves contributed (Chapters 4 and 5). There appears to be a contemporary need for a comprehensive review of γ-ray bursts and this is the reason for an extended Chapter 3. We have not included in Chapter 2 any material relating to γ-ray lines in solar flares – a very important subject in its own right – as we felt that it was outside the character of this book, dealing as it does with source regions exclusively beyond the solar system.