The popularity of the subject of gamma-ray astronomy has led to the need to update the material presented in the first edition, and this we are pleased to do.

The subject is in an exciting state in the lower energy region, below some tens of GeV, with the successful launch of the Gamma Ray Observatory in April, 1991. Already, sufficient data have appeared to show that, barring unforseen accidents, the subject will march forward at these energies. It is unfortunate that the Soviet GAMMA 1 satellite did not meet its design specifications – a reminder of the difficulties still inherent in satellite experiments.

The supernova SN 1987A continues to provide data of interest to the gamma-ray astronomer, and the results achieved so far have been included in this edition.

At the higher energies, advances have been less spectacular; indeed, there is some disappointment that many of the claimed TeV and PeV sources have still not been confirmed. Our view is that time variability of genuine sources married with some spurious signals probably accounts for the situation. Nevertheless, the subject is so important that continued, indeed enhanced, effort is needed.

The rate of publications in the field of gamma-ray astronomy at all energies is several times higher now than in 1985, when the manuscript for the first edition was turned in to the editors. Although we have made every effort to make the presentation in the second edition up to date (till the end of July, 1991), we apologise for inadvertent omission of any important results prior to that date.