Very few books, especially when written by many authors, have passed through any serious process of peer review. That this one has done so is due to the special way in which it was produced. All eighteen authors were asked to submit chapters which were then circulated to each of the others. Everyone was asked to write considered Commentaries on each chapter, with references where necessary, and these too were circulated to all the other authors. The purpose of the Commentaries was to be constructively critical and, where appropriate, to highlight areas of difference rather than to reach a consensus. Research workers, of course, always strive to reach a common truth in the end, but in the process their findings and ideas pass through a seemingly interminable period of discussion and argument; and during this time their enthusiasm is sharpened by the constructive cut-and-thrust of lively debate. Progress, indeed, comes as much from open discussion as from new discovery. New ideas and new results are regularly dipped in the acid of other people's opinions and findings, so that what remains is refined and likely to be reliably contributory. Our book is meant to be a contribution to that process.