The aims and scope of this book are set out in the beginning of the first chapter (unimaginatively labelled Introduction). So these prefacing comments are confined to acknowledging some of the help we and the other authors have received in putting this book together. The two of us are deeply indebted to the other 21 authors who have contributed to the book, both for the work they did and for their exemplary adherence to a rather fast production schedule. Individual authors have wished to thank both funding agencies and helpful colleagues who gave assistance of various kinds. This would have been an impressively long list, but we unkindly decided against including it.