This new edition is meant still to conform to the principles expounded in the above Preface. But eight years is about the doubling time of modern scientific knowledge and solid state physicists have not been idle in the interval. Most of the original text still stands, but several new sections have been added, to cover topics that have come into greater prominence lately or where there has been a significant shift of understanding or emphasis. I have also attempted to make reference in passing to a number of phenomena or fields of study that are relevant to the basic theory, even if they cannot be discussed in detail. In this way, the general scope of the book has been widened, to include, for example, something about magnetic and non-magnetic impurities, F-centres, surfaces, tunnelling, junctions, and type II superconductivity. But the general level of mathematical sophistication has not been raised, even though the technical formalism of advanced quantum theory is now becoming more commonplace in this field.

I am most grateful to many colleagues—especially to Bob Chambers here in Bristol and to Federico Garcia-Moliner in Madrid—for a number of detailed comments of which I have tried to take account in the new text. Bob Evans helped greatly by preparing a new index. And lest the reader may feel that absence from Cambridge has been a long period of exile, may I simply add that Bristol, too, is just as good a Ole to go to.