In 1975 we and four of our former colleagues wrote the textbook Principles of Tribology. That book was very successful both in the United Kingdom and overseas and it is still being sold today. However, 13 years after the first publication of the book, the publishers suggested that we should consider revising the text, to include the more important developments which had taken place in the meantime. Although we have remained active in tribological research, it was only when we started work on this revision that we realised that the developments which have taken place have been so extensive that they could not be accommodated simply by revision of our existing text. As a result, although we have reused some of our original material, the present book has resulted from a complete reappraisal of our ideas on the necessary content of a modern introductory textbook on tribology. As the title of the book implies, it has been our intention to write a book which will provide a thorough grounding in the principles of our subject for undergraduates and new research workers and will also act as a basic reference book for practising designers. In the areas of friction and wear, although there has been a great deal of high-quality research on the underlying physical and mechanical processes, there are no current, or indeed foreseeable, models which will allow quantitative predictions of friction coefficients or wear rates from basic physical, chemical and mechanical properties.