This text owes a great deal to current and past colleagues in the Department of the Environment. I am particularly grateful to Jim Smith for allowing me to see and use a large amount of otherwise unpublished material and to the members of the Evaluation Methodology Group under John Welsby’s chairmanship for the lessons learned in discussion of my own and others’ papers. Many others have contributed through their work or comments: if they were all listed fyere, the reader might wonder whether the author’s contribution went beyond the purely editorial. What, however, I have gained from their published or unpublished work will be apparent from the bibliography. The views expressed in this book are those of the author alone and do not necessarily reflect either those of his colleagues or of the Department of the Environment.