I am grateful to Malcolm Davies, James Diggle, Michael Reeve, Alan Sommerstein, and Martin West for reading the completed typescript and recommending improvements. Professor West also allowed me to see some unpublished work of the late W. S. Barrett which he is preparing for publication; while Almut Fries examined microfilms of the manuscript G in Göttingen, and Ranja Knöbl and Helmut Hofbauer made accessible other material available only in Germany. Malcolm Davies and Barbara Kowalzig gave me permission to cite their unpublished Oxford DPhil theses. My commissioning editor, Michael Sharp, and my copy-editor, Muriel Hall, have been of invaluable assistance in bringing the book to publication. I would also like to thank the staff of the Bodleian, Sackler, and Taylorian libraries in Oxford, the British Library in London, the Biblioteca Apostolica Vaticana in Rome, and the Biblioteca Laurenziana and Biblioteca Nazionale in Florence. Once again, it is a pleasure to record my gratitude to All Souls College, Oxford, where this book was written, for providing me with excellent working conditions and stimulating colleagues.