My editorial objective in this Series is to present to the scientific public a collection of texts that satisfies one of two criteria: the systematic presentation of a specialised but important topic within materials science or engineering that has not previously (or recently) been the subject of full-length treatment and is in rapid development; or the systematic account of a broad theme in materials science or engineering. The books are not, in general, designed as undergraduate texts, but rather are intended for use at graduate level and by established research workers. However, teaching methods are in such rapid evolution that some of the books may well find use at an earlier stage in university education. I have long editorial experience both in covering the whole of a huge field -physical metallurgy or materials science and technology - and in arranging for specialised subsidiary topics to be presented in monographs. My intention is to apply the lessons learned in 40 years of editing to the objectives stated above. Authors (and in some instances, as here, editors) have been invited for their up-to-date expertise and also for their ability to sec their subject in a wider perspective. I am grateful to Elsevier Science Ltd., who own the Pergamon imprint, and equally to my authors and editors, for their confidence, and to Dr. Rumen Duhlev of Elsevier Science Ltd. for his efforts on behalf of the Series. Herewith, I am pleased to present to the public the fourth title in this Series, on a topic of great current concern.